This sentence can be translated either as The ant eats a pizza or The ant is eating a pizza.
It said to type what you hear, and I had it correct!!! It said I typed it in English, which I did not.
The phrase was "the ant is eating the pizza." That's why it's "la pizza". If the ant was "eating pizza" (undefined quantity) or "some pizza", then it would be "de la pizza".
Being English I don't refer to "ants eating" but "on" or "got" etc.