Correct answer:
a. se lavan
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.
