¿tú y tus amigos estudiaron ayer? a. no, no estudiaron. c. sí, estudiamos ayer. b. sí, estudiar mucho. d. vas a estudiar mañana.

First of all, the response to this question must start with either the word sí or the word no. So we discard option d. On the other hand, we need to use the verb estudiar conjugated for the first person plural (nosotros - we) in the preterite tense, so this conjugation is estudiamos. Finally, the correct option is c:

Sí, estudiamos ayer.

well, if he says unos it is 1, if he says dos, it is 2, if he says tres, it is 3, if he says cuatro, it is 4