El alumno es serio. a. los alumnos son serio. c. los alumnos son serios. b. las alumnos son serios. d. los alumno son serios.

c. Los alumnos son serios.

Explanation:

Remember that in spanish when you are using the plural the verb and the whole sentece need to be changed into plural, by adding an S at the end of the verb and at the end of teh subject, and depending of if it is female or male the subject you will use las, or los, in this case it is male so we use Los alumnos son serios, and that would be the correct option.