The dictionary lists different uses of antworten. One of them is transitive in the sense that it takes an accusative object that can become the subject under passivisation (marked in bold in the following examples). The accusative object indicates the content of the reply. A dative object, specifying who receives the reply, is optional.
Was hast du (ihm) geantwortet? (accusative object, active)
Was wurde (ihm) geantwortet? (subject, passive)
Es soll nichts geantwortet werden. (translation of Latin nihil esse respondendum)
Er hat wieder nur einen Satz geantwortet.
For further examples of transitive antworten, see the entry for antworten in "Elektronisches Valenzwörterbuch deutscher Verben".
But there are also intransitive uses, where no accusative object is possible. These should be listed under [Vi] in the dictionary, where the entry is cut off in the question. In the following example, antworten takes the meaning of "react" and occurs solely with a prepositional object headed by mit; there are no nominal (dative or accusative) objects.
Er antwortete mit einem Nicken.
The prototypical intransitive example with only a dative object (specifying who receives the reply, marked in bold) would be the following. As this is the most common usage, it's the one that gets drilled into learners' heads ("antworten takes the dative!").
Hast du ihm schon geantwortet?