The answer is C because estudiantes means students and students are a plural word. Furthermore, you use the word "an" as the conjugation when something is plural. Therefore, the best conjugation that represents that is guardan. Additonally, guardar means to save and put.
Here are all the conjugations of a present tense word in Spanish: I will use the example comprar.
Compro = I buy
Compras = You buy
Compra = He/she buy
Compramos = We buy
Compran = They buy
So basically if something ends with something, it represents a unique pronoun for that verb.
