아이가(the child) 숙제를(the homework/his homework) 안(not) 했어요(did do)
so, the correct answer is "the child did not do the(his) homework", not "the child did not do their homework". OK??? not 'their' .......
"His" is either an old-fashioned sexist possessive adjective or it unreasonably presumes the child is male. In informal English, "they," "them," and "their" are commonly used to refer to a singular person whose gender is not known or not specified.