The correct option is
A Collective Noun
Option A: Collective nouns refer to a group of things or living beings, and are considered as a singular entity. In the given sentence, the noun 'flock' refers to a group of sheep. Hence it's a collective noun and option A is correct.
Option B: Abstract nouns
refer to things that cannot be perceived through any of our senses
(touch, see, taste, smell, hear). They are emotions, activities or states of being.
For example, 'happiness', intelligence', etc. The 'flock' isn't abstract: it is a group of living beings that can be seen.
Option C: Uncountable nouns refer to things that cannot be counted. 'Flock' is not uncountable. It's collective and is regarded as a singular form, but it can also be counted as one flock of sheep, two flocks of sheep, etc.
Hence options B and C are incorrect.
Option D is wrong because the answer lies in A.