The correct option is B Between visiting the pristine beaches and mountain peaks, travellers to the Caribbean would be spoilt for choice.
The preposition ‘between’ is used to talk about two contrasting ideas. Here, the sentence talks about two contrasting ideas i.e. ‘visiting the beaches’ and ‘visiting the mountain peaks’, that’s why it’s correct. However, the other option uses ‘between’ to talk about a group of similar ideas, therefore, it is incorrect.