A direct object pronoun would be "it", as "it" would refer to "la cena".
To answer with a DOP, you just take the "la" from "la cena" and attach it to the infinitive.
The answer would be, either:
1) Si, voy a prepararla.
or
2) No, no voy a prepararla.
You can also take the "la" and place it before "voy" like this:
1) Si, la voy a preparar.
or
2) No, no la voy a preparar.
