Which phrase is correct: Two students _____________ their professor about the possibility of obtaining a good grade in class?

The phrase "Two students ask their professor about the possibility of obtaining a good grade in class" correctly uses the verb "ask" in the present tense, which aligns with the plural subject, "Two students." In English grammar, verbs must agree in number with their subjects; since "students" is plural, a plural verb form is necessary.

Using "ask" indicates a present action, suggesting that the students are currently in the process of questioning their professor. This choice effectively conveys the ongoing nature of the inquiry about grades.

In contrast, the other choices do not fit grammatically. "Is" and "was" would imply a singular subject, which does not correspond with "Two students." "Could" also does not fit well in this context, as it would typically require a more complex structure, such as "could ask," but does not serve as a main verb on its own in this sentence. Thus, the chosen phrase maintains the agreement and communicates the intended action correctly.