Right answer:
b. volvamos
Volvamos is the conjugation of the verb volver for the first person plural in the subjunctive mood. We use the subjunctive to express desires, doubts, the unknown, the abstract, and emotions. As in the indicative, the subjunctive mood includes the same verb tenses, including the perfect, the past, and the future. Subjunctive sentences often have parts linked by a relative pronoun (que, quien, como). Finally:
Los profesores insisten que nosotros volvamos a tomar clases de verano.