(A) Incorrect.

This option strengthens the argument but is not an assumption.

The option says that smokers are less likely than nonsmokers to have nutrient-rich diets. If so, I have a reason to believe that increasing the nutrients in their diets will help them. Thus, I have more belief in the conclusion. (If smokers were already taking nutrient-rich diets, I’d have less reason to believe that increasing the nutrients in their diets would improve their lung function.)

Given that the option strengthens the argument, the next step is to evaluate whether the negation of the option breaks down the argument.

Negation: Smokers are as likely as or more likely than nonsmokers to have nutrient-rich diets.

The negation weakens the argument since it indicates that smokers already have enough nutrients and thus more nutrients may not help them.

However, the negation doesn’t break down the argument since more nutrients may still improve smokers’ lung function, given the correlation mentioned in the first statement of the passage. The negation doesn’t make the argument baseless.

(B) Incorrect. 

This option strongly strengthens the argument but is not an assumption.

The option very strongly indicates that increasing the nutrient intake will improve the lung function of smokers.

How does it indicate so?

We know from the passage that the lung function of smokers is significantly worse, on average, than that of nonsmokers. Now, this option tells us that the lung function of smokers whose diets are rich in those vitamins and minerals is generally better than that of nonsmokers with comparable diets.

Thus, it seems that diets rich in those vitamins and minerals have the potential to make the lung function of smokers even better than that of nonsmokers.

Negation: The lung function of smokers whose diets are rich in those vitamins and minerals is generally the same as or worse than that of nonsmokers with comparable diets.

This option is a mild weakener; it doesn’t even come close to breaking down the argument.

Even if the lung function of smokers with nutrient-rich diets is worse than the lung function of the nonsmokers with comparable diets, we still have a reason to believe in the reasoning in the argument. Increasing the nutrient-intake may still improve the lung function of smokers; it may not make their lung function at the same level as the lungs of nonsmokers; however, as long as their lung function improves, the argument holds.

(C) Incorrect. 

This option has NO IMPACT on the argument.

Whether people with nutrient-deficient diets have other health problems or not has no relevance to the argument. The argument is concerned ONLY with lung function.

(D) Incorrect. 

This option strengthens the argument but is not an assumption.

This option says that diet change can typically improve lung function as much as or more than stopping smoking will.

Thus, this option suggests that diet change is an effective way for smokers to improve lung function. Therefore, this option strengthens the argument.

Negation: Stopping smoking will typically improve lung function more than any diet change can.

The negation says that stopping smoking is going to help the smokers more than any diet change can w.r.t. improving lung function.

While the negation weakens the argument mildly, it doesn’t come close to breaking down the argument since even if diet change is less effective than stopping smoking, diet change can still help a lot. Thus, the argument still is very believable.

(E) Correct. 

This option strengthens the argument and, on negation, breaks down the argument (i.e., makes the argument baseless)

The option says that smoking does not introduce certain chemicals into the body. Which chemicals?

Chemicals that stop the helpful nutrients from entering the bloodstream.

If these chemicals are not introduced by smoking, we have a reason to believe that increasing the nutrient-intake will help the smokers since these nutrients will be able to enter the bloodstream and thus help the lung function.

This option, thus, strengthens the argument.

Negation: Smoking introduces into the body chemicals that prevent the helpful vitamins and minerals from entering the bloodstream.

If smokers have chemicals in their bodies that do not allow the nutrients to enter the bloodstream, then even if they increase their nutrient intake, they’ll not have those nutrients in their bloodstream. If they don’t have these nutrients in their bloodstream, we don’t have a reason to expect better lung function.

Thus, in this case, the argument becomes baseless, i.e., we don’t have a logical reason to argue for the conclusion on the basis of the premises.

Let’s take a moment to understand how the argument becomes baseless in the presence of the negation of this option.

Premise: The higher the level of certain vitamins and minerals in the bloodstream, the better a person’s lung function.

Negation: Smoking introduces into the body chemicals that prevent the helpful vitamins and minerals from entering the bloodstream.

Conclusion: one way for smokers to improve their lung function is for them to increase their intake of foods that are rich in these helpful vitamins and minerals.

Given that nutrients in smokers’ bodies cannot reach the bloodstream, how can we expect that increasing their nutrient intake will help their lung function? What we believe is that higher level of nutrients in the bloodstream helps the lung function. If the nutrients don’t even enter the bloodstream, we don’t have a reason to expect improved lung function. Not having a reason to believe in the conclusion is what we call an argument’s becoming baseless.