答对 is a resultative verb compound (V + 对 as complement) meaning 'to answer correctly'. The structure is 答 + 对, and you can insert 得/不 between the two syllables to form the potential complement: 答得对 ('can answer correctly') and 答不对 ('cannot answer correctly'). The object can be placed after the whole compound (答对问题) or before it in a 把-construction (把问题答对了). The simple gloss 'reply; answer' misses the core meaning of 'correctness' — if a learner translates 'answer' back into Chinese, they'd likely produce 回答 instead.