Which of the following medications is used specifically for chemotherapy-induced nausea and vomiting?

Aprepitant is the correct choice because it is a medication that specifically targets chemotherapy-induced nausea and vomiting (CINV). It is a substance P/neurokinin-1 (NK1) receptor antagonist, which works by blocking the action of a neurotransmitter involved in the vomiting reflex, thereby providing effective relief from nausea and vomiting caused by chemotherapy.

Compazine, while it can be used to treat nausea, is primarily an antipsychotic medication and is not specifically indicated for CINV. Olanzapine, although it can help with nausea in some situations, is primarily an antipsychotic as well but has shown effectiveness in CINV as an adjunct treatment rather than a first-line agent. Darbepoetin alfa is a medication used to treat anemia, particularly in patients with chronic kidney disease or those undergoing chemotherapy, but it does not have a role in managing nausea or vomiting.

Thus, Aprepitant stands out as the medication with a dedicated indication for preventing and treating nausea and vomiting associated with chemotherapy.