Addiction prevents people to quit smoking.
Reduction in nicotine content per cigarette.

Lower the nicotine intake, easier to quit --> Proportion of people attempting to quit smoking will increase.

We have to find an option that must be true in order for the conclusion to hold true.

a. The strength of a person's nicotine addiction depends only on the amount of nicotine that person regularly consumes.
Incorrect. Negate A: Strength of person's nicotine addiction does not depend only on the amount of nicotine that person regularly consumes. Negating A does not have any effect on the conclusion.

b. Most of the cigarette smokers in Tarnia would quit smoking if they were not addicted to nicotine. -
Incorrect. Out of context. Option presents info about people not addicted to nicotine.

c. It is unlikely that many Tarnians who have not already become addicted to nicotine will become addicted to nicotine at some time over the next five years. - Incorrect. Out of context. Same error as in B.

d. Tarnian smokers addicted to nicotine will not generally compensate for decrease in the amount of nicotine per cigarette by smoking more cigarettes. -
Correct. Negate D, conclusion breaks down.

e. All cigarettes currently sold in Tarnia contain the maximum amount of nicotine permitted by law.
- Incorrect. Irrelevant.

Answer: D