grammar (common mistake)

The correct one is [a], because the word [wrong] is an adverb itself, you need to use an adverb to modify a verb, in this case [answer], [wrongly] is also an adverb, but it means another thing.

2008-07-26 01:22:56 補充：
Wrong:
ad.
1. 錯誤地
He guessed wrong.
他猜錯了。
2. 邪惡地,不正當地

wrongly:
ad.
1. 錯誤地,不正確地;不恰當地
She supposed, wrongly, that the other two agreed with her.
她錯誤地以為其他兩人是贊同她的。
2. 不正直地,不公正地

c If you answer this question wrongly

Wrongly is the adverb used to modify the verb 'answer'.