Final answer:
Blocking serotonin and dopamine receptors in the CTZ will have an antiemetic effect, and the correct answer is D) A and C only.
Step-by-step explanation:
Blocking either serotonin or dopamine receptors in the chemoreceptor trigger zone (CTZ) will have an antiemetic effect. Serotonin antagonists, like ondansetron, are known for their use in preventing nausea and vomiting associated with chemotherapy, radiotherapy, and postoperative situations. Dopamine antagonists, such as metoclopramide, are also used to prevent nausea and vomiting. Therefore, the correct answer to the question 'Blocking which of the following neurotransmitter receptors in the CTZ will have an antiemetic effect?' is D) A and C only.