"She responded to him."
Translation:Ela respondeu a ele.
"Ela o respondeu" é correto tambem. O verbo "responder" é transitivo e intransitivo. Por isso, pode tomar um objeto direto. "Ela o respondeu" is also correct. In Portuguese the verb "responder" is both transitive and intransitive. This means it can take a direct object or be used with a preposition. "Answered" in English is transitive. It can take a direct object. "I answered him with a smile." But "respond" is only intransitive in English. "I responded to him warmly in my letter." "Answered him" but "responded to him".
That is true. But it no longer means "respond" in this case. When used with the preposition "to" the meaning changes. "John answers to Mary" means John is Mary's subordinate. "John will have to answer to Mary for his crimes," means that when John is brought to justice he will be held accountable by Mary.
Not sure if Duo has fixed the accepted answers but, "ela respondeu-lhe" was accepted (4 years later :D).
From what native speakers have told me, it is grammatically correct but no one ever uses the indirect article in speech