"He visits them."
Translation:Él las visita a ellas.
Yeah that is odd that it would accept both. I just tried a while to find the sentence in the Object Pronoun section but it never gave me the sentence in English to test my Spanish translation on. In a two choice it only gives las/los to test if you know what matches "ellas". It also used "Él las visita a ellas" in the Spanish to English test.
It rejected your answer, because you didn't conjugate visitar quite right. You used the 2nd person singular, but it needed to agree with the subject, Él. Él los visita would have worked. Los visita is also accepted, but it is understood that the Él had already been established in context with this use.