Las muchachas el pelo todos los días. a. se lavan b. se lava

This sentence is given in reflexive form because it uses the reflexive pronoun that matches the third person plural, which is se. We use Reflexive Pronouns with a verb to point out that a person carries out an action to or for himself or herself. On the other hand, lavan matches the third person plural in the simple present. Finally:

Las muchachas se lavan el pelo todos los días.