Aside from that, why isn't "tomato and banana are fruit" accepted as an answer? The word "fruit" can be the plural form, as well as "fruits."
"Tomato and banana are fruits" is not English idiom. The nouns need articles if they are singular and the usual form of such a general statement is in the plural, i. e., "Tomatoes and bananas are fruit." "Fruits" would be OK, but I would naturally say "fruit."
"The tomato and the banana are fruits" sounds like a line from a documentary or lecture.
Scientifically, tomatoes are fruit, as they are the fertilized ovary of a flower.
Incorrect English. Should be : tomatoes and bananas are fruit