An example of a legal stimulant is
a. crack.
b. cocaine.
c. caffeine.
d. amphetamine.

The correct answer is C. Caffeine

Explanation:

Answers A and B are wrong because both are illegal in many states and countries (and they're the same thing. "Crack" is a nickname for cocaine.) Amphetamine is a highly addictive drug that is way too strong for many people and can cause easily overdoses for first-time users and is also outlawed.

Caffeine, however, is very legal. We know this since it's a very common ingrediant in the every-day coffee use that many people consume.