Fact 1: The higher the level of certain vitamins and minerals in the bloodstream, the better a person's lung function, as measured by the amount of air the person can expel in one second.
Vitamins and minerals in blood stream helps lungs to perform functions.
Fact 2: The lung function of smokers is significantly worse, on average, than that of nonsmokers.

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.
Smokers can improve lung function by intake of vitamins and minerals.

A. Smokers are less likely than nonsmokers to have diets that are rich in vitamins and minerals.
If true, This additional information strengthens the argument. Incorrect.
B. The lung function of smokers whose diets are rich in those vitamins and minerals is generally better than that of nonsmokers with comparable diets.
If true, This additional information strengthens the argument.As taking V&M will help the smoker to enhance their lung functions. Incorrect.
C. People whose diets are deficient in those vitamins and minerals do not typically have other health problems in addition to diminished lung function.
Not related to Smokers ,so, It is irrelevant.
D. Stopping smoking will not typically improve lung function more than any diet changes can.
This is out of scope.As it is introducing new way to enhance lung function.
E. Smoking does not introduce into the body chemicals that prevent the helpful vitamins and minerals from entering the bloodstream.
What if smoking introduce body chemicals that prevent the V&M from entering blood stream? Then the conclusion will not stand true.hence, It is a necessary assumption for argument.correct.