The correct answer is B, hallucinogens.
Antidepressants are most definitely legal, tons of people are on them. It is illegal to sell them, though (unless you are a pharmacist).
Painkillers are the same way, and many times otc.
Antihistamines are similar to painkillers, mostly used for allergies, and the name brands are extremely common; Zyrtec, Claritin, Allegra and Benadryl.
Meanwhile, hallucinogens are drugs such as LSD, PCP, ketamine, mushrooms. These wildly impact human mind and body and are therefore illegal.
(I would like to mention that ketamine is an antidepressant, but now identified more so as a hallucinogen due to the fact that it's effects last about an hour. This has caused it to become a party drug called "crazy k" which is often times laced with cocaine.)