Which of the following sentences uses "must" correctly?

Using "must" correctly in a sentence conveys necessity or obligation. In this instance, both of the sentences provided in options A and C effectively demonstrate this usage.

In the first sentence, "You must finish your homework before going out," the term "must" indicates a strong obligation to complete homework prior to engaging in recreational activities. This reflects a necessary condition that emphasizes responsibility.

In the second sentence, "You must not use your phone during the exam," the phrase conveys a clear prohibition, communicating that using a phone is not permitted in that particular context. This also adheres to the rules of obligation, albeit in a negative form, indicating that there are consequences for not adhering to that guideline.

Since both sentences correctly employ "must" to express necessity or obligation, the answer that includes both A and C is indeed the most appropriate choice.