What is the correct form for this question: _________ you taking a cruise to commemorate your wedding anniversary?

The correct form of the question is "Are you taking a cruise to commemorate your wedding anniversary?" This is because the subject "you" requires the present tense form of the verb "to be," which is "are." In English, the verb "to be" must agree with the subject in both number and person. Since "you" is second-person, "are" is the appropriate choice.

In the context of the other choices, "is" would be used with a singular third-person subject, "was" refers to the past tense, and "could" suggests a possibility rather than forming a question about a current action. Therefore, "are" is the only correct verb form to make a present tense inquiry regarding plans for the future involving the subject "you."