Question

Fill in the blank with the correct form of the imperfect tense.

Answer 1

Los alumnos (estudian) para el examen.   The students (are studying) for the exam.

*Not sure if you're supposed to put this sentence in past/present tense but I used present above.

If the students (studied) for the exam, then the answer would be:
Los alumnos (estudiaron) para el exam.

Answer 2

Estudian =study
Estudio=i study (present tense)
Estudiar=i will study
So estudian is the correct answer all you have to do for questions like these is study the endings ex.o,as,an