Welcome to another terrible multiple choice question. They get worse, don't they?
Obviously all of those options would fit. It is also impossible to test words like school, university and college because they overlap,and because every English-speaking country uses them differently.
I agree with the wonderfully named Hamlet D'Arcy, though. I think that the daft person who wrote this was trying to rule out 'university' and 'college' by using the word 'pupils'. 'Pupils' suggests schoolchildren rather than college- or university-aged students, so I'm guessing that they want you to put 'school'.
NB Further to Mr H D'A's comment about the term 'pupil'. In fact, we don't use 'pupils' in British schools: it is very outdated. You might come across it in a historical context - for example, in a biography of a nineteenth-century schoolteacher - but it is rare to hear it in modern contexts. These days, even schoolchildren can be referred to as 'students'.