Image Provided - Why is 는 used in only one if the following answers but both are correct?

First of all, I must say that flashcards do not provide sufficient context which is particularly important for language learners. You should read multiple sentences that relate to one another rather than trying to understand separate sentences. Books are usually better materials.

For that English sentence, both

과일 중에서 수박이 제일 큽니다.

and

과일 중에서는 수박이 제일 큽니다.

are correct. It is just that the second sentence can mean something more. What is it? Please refer to this first. I can think of at least two possible answers:

1. The preceding sentence was something like this:

이 중에서 어느 것이 제일 큰지 아십니까?

or

식물 중에서는 세쿼이아가 제일 큽니다.

2. The following sentence is going to be something like this:

식물 중에서는 세쿼이아가 제일 큽니다.

or

(그리고) 과일 중에서는 개구리밥이 제일 작습니다.

Without the context, however, there is no way for us to know the speaker's intention.