"An animal is eating a black apple."
Because you need to consider “is -ing” as a whole. It usually means something that is happening right now. If you really want to use « est », you can say « Un animal est EN TRAIN de manger une pomme », meaning “is eating AT THE MOMENT”, but it add a lot of useless weight to the sentence.
Why the female noire and not noir? I used noir to describe apple before and it accepted it as right.
Okay, I could understand using black if it were a number of other things, but apples are red! Red and black have been used so far, why not use it with the food that could actually be red? I don't see how an animal would want to touch a black apple.;.
I think the exercise is more to teach structure and adjective matching, rather than laying out literal situations. And la pomme is an easy word with which we can learn right away, instead of teaching new words and concepts simultaneously.