Answer:
1. Pedro le compra un regalo a su hermana.
2. Le escribo una carta a mi abuela.
3. Los estudiantes le tiene que entregar la tarea a la profesora.
4. Marta le va a leer el libro a sus primos.
Explanation:
Because we must identify the person, the gender, the time, the number and the verb in each sentence, so we will be able to identify who performs the actions described with the information they provide us and we can build sentences that are consistent with what is being expressed.