Right answer:
a. nuevo
Nuevo is an adjective that translates into new. An adjective is a word that gives us more information about a noun or pronoun. These are used to describe some quality of a noun like size, color, or shape of a person, an animal, a thing, or a place. So this adjective tells us that the student is new at a school. Finally:
Él es un alumno nuevo (He is a new student)