What would happen if Isabel and her family had gotten out on the beach they landed on? They were having a party that the whole family was invited to
they would be sent back to Cuba
they would have been sad because they did not have any swim wear to enjoy the beach
they would have to build a house

I believe it is they would be sent to Cuba.

Explanation:

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.