Correct answer:
A. interrumpen
Since we can replaced los estudiantes by ellos, this sentence stands for the third person plural, which is they in English. So interrumpen is the correct conjugation of the verb interrumpir for the third person plural in the simple present. The simple present tense is used to talk about habitual actions or routines. This tense is also used to describe things that happen now or in the near future. Finally:
A veces los estudiantes interrumpen al maestro.