For this case we can translate the sentence given as:
She ____________________ a student at Martí School.
We must complete the sentence with the proper conjugation of the verb "to be" (ser in Spanish)
We have the verb "ser" indicates existence. By conjugating it indicatively in the present tense we have:
Yo soy
Tú eres
él/ella/eso es
nosotros somos
vosotros sois
ellos son
So, the appropriate option is option C.
ANswer:
Ella es una alumna en el Colegio Martí.