Answer:
Cuando era niña me gustaba _X_ peinar a mi papá.
No reflexive pronoun is needed.
Explanation:
In this sentence, you do not need any reflexive pronoun. It would be grammatically incorrect if you add one.
Reflexive pronouns are used with reflexive verbs in Spanish (such as peinarse, maquillarse, lavarse) to indicate actions that are performed on oneself. That is, the subject and the object of the action are the same. However, in the sentence Cuando era niña me gustaba peinar a mi papá, the subject is "yo" and the object is "a mi papá" ((I liked brushing my dad's hair). Therefore, as the subject is not performing the action on himself but on another person, you do not need to use a reflexive pronoun.
