Right answer:
b. te
In this exercise, I ask you a question that translates into Do you write me a letter? So me is the indirect object pronoun that matches the first person singular. An Indirect Object Pronoun is a person that receives the action of a verb indirectly. Its purpose is to tell you to whom or for whom something is done. So we need to give a response to this question using the indirect object pronoun that matches the second person singular, which is te. Finally:
- ¿Me escribes una carta?
- Sí, te escribo