Final answer:
The correct choice for this fill-in-the-blank sentence is B) does, leave. This is because 'boat' is a singular subject which requires singular verb form 'does leave'.
Step-by-step explanation:
The correct choice is B) does, leave. This sentence requires a singular subject-verb agreement. The subject 'boat' is singular, so the verb 'leave' needs to fit with it. To make this singular, we use 'does leave' instead of 'do leave' which is used for plural subjects. Remember, in these types of questions you need to match the verb form to the subject's number - singular or plural. Hence, The sentence would be "When does the next boat leave?"
Learn more about Subject-Verb Agreement