“Compro la gasolina en la estacion servicio”
Substitute “la gasolina” with a direct object pronoun, and explain why you used the pronoun you did.
“La compro en la estacion de servicio”
In this case, we use the direct object pronoun "la" because it refers to the feminine noun "gasolina." In Spanish, direct object pronouns must agree in gender and number with the noun they replace. Since "gasolina" is a singular feminine noun, we use the feminine singular pronoun "la" to replace it.