Right answer
c. Sí, unos alumnos van a la escuela a pie.
The question ¿Los alumnos van a la escuela a pie? translates into Do students go to school walking? So the response to this question must be related to the question. In this way, the only option that meets the requirements is c. becuase affirms that in fact they go to school walking. In this way:
Sí, unos alumnos van a la escuela a pie translates into Yes, some students go to school walking