Is it because you’re being asked how many ba of birds there are instead of how many wa?
For those unfamiliar, Japanese uses what’s called “counter words,” words that come after a number when counting certain things.
However, in the above question, the teacher decided to forego using wa and used ba instead to count the chicks… for two out of three times.
While birds were counted using ha in the past, it’s rarely done today.
Even if birds were counted using ha (which they’re not really anymore), the teacher’s corrected answer of go-ba would still be wrong.