Translation:Neither the mouse nor the bird eats the apple.
I get bitten by that too. I think "eats" is grammatically correct, but as you said, "eat" is in practice common enough (and may in fact be accepted; language ultimately is democratic and what most people say IS eventually accepted as correct usage, despite what grammarians try to tell us).
But yeah, it's frustrating as heck to know the answer and get market wrong b/c of an English typo or grammar error (or, even less). I think Duolingo should be less forgiving of our errors with Vietnamese diacritics (which actually DO change the meaning) and more forgiving for English spelling and grammar errors and typos.
The document Nguyen Đinh Hoa (1957) Classifiers in Vietnamese, (WORD, 13:1, 124-152, DOI: 10.1080/00437956.1957.11659631) which I found at https://www.tandfonline.com/doi/pdf/10.1080/00437956.1957.11659631 says that both trái and quả can be used as a classifier for a fruit, or a round, spheric object (on page 150).
It also tells me that the English collective noun for kittens is 'kendle' - always learning.