Answer: A:) When an infinitive verb follows a conjugated verb.
Explanation: in indicative sentences that use infinitives, you can either attach the direct object pronoun to the end of the infinitive verb or put it before the first conjugated verb.
Voy a comprar un coche.
I'm going to buy a car.
Direct object is car.
Direct object pronoun is "lo", masculine and singular.
Voy a comprarlo.
I'm going to buy it.
Lo voy a comprar.
[tex]\textit{\textbf{Spymore}}[/tex]