¿vas a llamar el taxi?
a. sí, voy a llamar
b. sí, voy a llamaria
c. sí , voy a llamar el taxi

A. Sí, lol voy a llamar

Explanation:

Direct objects are lo, la, los, or las. Choice C has none of those, so that can be eliminated. Choice B has la at the end of llamar to make llamarla, but El taxi is masculine, not feminine, so it can't be la. So the only choice left is A because it uses the direct object lo (since el taxi is masculine) and it's in front of the verb voy.