"Voi ăn cỏ không" should be right.
Please fix, this is obvious generic elephants eating generic grass, not specific elephants.
You need to have có in the sentence. "Voi có ăn cỏ không?" is acceped.
No you don't. In fact, in some cases 'có...không' isn't right, and you use just không
3rd answer is singular. the given sentence is plural 'elephants'. perplexing.