a sentence
When I've been to Mustique and Barbados, I _____________________  (see) all the islands of the Caribbean.

In which tense would you put the verb (see) and why?

It's an easy question for a teacher or a native speaker, so why don't you try to give an answer and explanation, and then we can then give you feedback?

When I've been to Mustique and Barbados, I __have seen___________________ (see) all the islands of the Caribbean