Final answer:
The most common side effect of ondansetron, also known as Zofran, is headache.
Step-by-step explanation:
The question is regarding the most common side effect of the medication ondansetron, which brand name is Zofran. Ondansetron is frequently used to prevent nausea and vomiting associated with cancer treatments such as chemotherapy and radiation therapy, as well as after surgery. While it is used to combat nausea, nausea is not a common side effect of the drug itself.
The most common side effect of ondansetron is actually headache. Though other side effects like hypotension, QT prolongation, and constipation can occur, they are less common. When considering the options given in the question, headache (A) would be the correct answer.