Right answer:
Participan
Participan is the conjugation of the verb participar for the third person singular in the simple present. The simple present tense is used to talk about habitual actions or routines. This tense is also used to describe things that happens now or in the near future. So this sentence becomes:
Ana y Susana participan en la clase de ciencias.
Whose translation is:
Ana and Susana participate in the science class.