Which phrase correctly completes this sentence? ayer, mi hermano menor la cabeza con una pelota.
a.) se golpea
b.) nos golpearon
c.) te golpeaste
d.) se golpeó

Se is the reflexive pronoun that matches the third person singular. In Spanish, we use Reflexive Pronouns with a verb to point out that a person carries out an action to or for himself or herself. So My younger brother (mi hermano menor) carries out the action to hit with the ball for himself. On the other hand, golpeó is the conjugation of the verb golpear that matches the third person singular in the preterite tense. Finally:

Ayer, mi hermano menor se golpeó la cabeza con una pelota.