A moist, tropical region at high risk for hurricanes are the Caribbean Islands.
Explanation:
The region type of tropical and moist, as the name suggests, seems to be in plural form. Since it is in plural 'are' is the suitable word for the given sentence. On the other hand, it matters based on the subject, of course, 'is' or 'are'. Using 'is', makes the sentence grammatically incorrect. If the sentence has to be modified to a different voice by using 'are', it continues to make sense by being grammatically correct.