Minor point— in sentence building exercises, the first word of the sentence shouldn’t be written with a capital letter in the word box — this alerts the user that it’s definitely the first word. Of course some words must always be capitalized (proper nouns), but not elephants!
In this sentence the plurality of हाथी is indicated in the plural form of the verbs खाते हैं . If we were just talking about one elephant, it would be हाथी बड़े केले खाता है .
This is similar to English words with identical singular and plural forms, like "sheep". "My sheep eats apples" would refer to a singular sheep, but "my sheep eat apples" would refer to multiple sheep.