The correct option is
C Abstract Noun
Option C: 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.
In the given sentence, 'hard work' refers to 'a great deal of effort'. It's an activity that involves a good amount of effort and cannot be perceived through the senses. Hence it's an abstract noun and option C is correct.
Option A: Collective nouns refer to a group of things or living beings, and are regarded as a singular entity. For example, 'a school of fish'. The noun in the given sentence does not refer to any group.
Option B: Concrete nouns are the opposite of abstract: they can be perceived through the senses. 'Hard work' cannot be perceived through the senses. So, it's not a concrete noun.
Hence options A and B are incorrect.
Option D is wrong because the answer lies in C.