need answers to this spanish Conjugation. no website although i tried spanishdict to get answers. it says fill in the blank with the correct Conjugation of the verb that's in parenthesis. it's 10 questions:​

When could you attach the direct object pronoun to an infinitive verb? a: when an infinitive verb follows a conjugated verb. b: when an infinitive verb ends in a question. c: when a stem-changing verb ends in a question. d: when a stem-changing verb follows a conjugated verb.