Which drug causes vomiting in cats?

Right on!  You chose A - Xylazine, the correct answer!

Xylazine is used as an EMETIC in CATS - i.e., it causes vomiting.

Three things to remember about xylazine:

1. Cattle are EXTREMELY SENSITIVE to xylazine. For example, cattle need only 1/10th the dose of xylazine used in horses.
2. Reversed with yohimbine, atipamezole, or tolazoline.
3. Pretreatment with atropine can decrease bradycardia and hypersalivation seen with xylazine in cattle.

Two ANTI-emetics in cats are diphenhydramine (Benadryl®) and metoclopramide.

Refs: Papich, Saunders Handbook of Vet. Drugs: Sm & Lg An., 4th ed. p. 850 and Bassert and Thomas, McCurnin’s Clinical Textbook for Veterinary Technicians, 9th ed p. 900.