The direct object pronoun would be "la" because la gasolina is feminine. In a sentence you could say
La compro en la estacion de servicio.
Direct object pronouns are placed before the verb generally.
Please tell me the five vocabulary words, and then I can reply with the answer :)
Okay the first one is ALTOS, the second one is SERIAS, and the last one (I'm pretty sure) is SIMPATICA.