Given there's no past tense in the Vietnamese, then DO or DID should be correct when translating from Vietnamese to English. Unfortunately the system is too rigid or too simple and DID is marked is incorrect.......... the Level of English here is very very disappointing.
Where did they hear it? Should be the accepted answer. And though technically no past tense, thấy would imply so being that it indicates a successful ability to do something, in this case hearing something.