You need to use 'beantwoorden' with an direct object. Since there is no object in this sentence, you cannot use 'beantwoorden'.
xMerrie is pointing to the fact that the verb "beantwoorden" is transitive, whereas "antwoorden" is intransitive.
This is actually the function of the "be-" prefix in Dutch: it indicates that the verb is acting on a direct object, thus making it transitive.
Ik woon. = I live (as in reside).
Ik bewoon het huis. = I inhabit the house.
Ik jaag. = I hunt.
Ik bejaag hem. = I hunt after him.
I antwoord. = I answer.
Ik beantwoord hem. = I answer him.
Ik loop. = I walk.
Ik beloop het tapijt. = I walk on the carpet.