Countable nouns name things or beings that can be counted. For example, one apple, three bears, etc.
Uncountable nouns cannot be counted, because they refer to quantity or mass. For example, some milk, some apple juice, etc.
Collective nouns refers to a group of things of the same type. For example, 'a swarm of bees', 'a pack of wolves', etc. Collective nouns refer to a group as a singular being.
Abstract nouns refer to things that cannot be perceived through any of our senses (touch, see, taste, smell, hear). They are emotions or states of being. For example, 'happiness', intelligence', etc.
Option A: In the given sentence, 'pupils' refers to a number of students. They can be counted, as mentioned in the sentence 'twenty pupils'. Hence it's a countable noun and option A is correct.
Options B, C and D:
'Pupils' cannot be uncountable as it refers to twenty students already.
It cannot be collective because it does not refer to the pupils as a singular form. (For example, 'a class of pupils') The sentence expresses individual pupils, hence they have been counted as twenty.
Pupils are students, human beings that can be perceived through all our senses. The noun cannot be abstract.
Hence these options are incorrect.