Answer:
The correct answer is se divierten.
Step-by-step explanation:
This is the case of a reflexive pronoun. This type of pronoun indicates an action that falls on the person who is carrying it out. In other words, it is done by the subject for himself: "The fish have fun."
Depending on the personal pronoun, a reflexive pronoun will correspond. Let's see :
Yo me divierto
Tú te diviertes
Él se divierte
Ella se divierte
Usted se divierte
Nosotros/as nos divertimos
Ellos / Ellas se divierten
Ustedes se divierten