The correct option is
B Collective Noun
Option C: 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.
In the given sentence, 'fleet' refers to 'a group of ships'. Hence it's a collective noun and option C is correct.
Option A: 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.
'Fleet' does not refer to any emotion or state of being.
Option B: Uncountable nouns cannot be counted, because they refer to quantity or mass. For example, some milk, some apple juice, etc.
The noun 'fleet' is collective. It refers to a group of ships. It's taken as a singular entity and is hence a countable noun. It can be confused as uncountable, but it isn't a mass noun. When we say 'a fleet' we're referring to one fleet. (It can also be two fleets, three fleets, etc.)
Hence options A and B are incorrect.
Option D: This option is not right because the answer lies in option C.