The correct option is
A Abstract Noun / Uncountable Noun
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.
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.
Abstract nouns cannot be counted, hence they can also be called uncountable nouns.
Concrete nouns refer to things that can be perceived through our senses.
Option C: In the given sentence, 'strength' refers to the 'state of being strong'. It's an abstract noun and cannot be counted. Hence Option C is correct.
Option A: 'Strength' cannot be perceived through our senses, it's an abstract state of being. Hence it isn't a concrete noun.
Option B: Abstract nouns cannot be counted, they refer to an emotion or a state. So, 'strength' is not a countable noun.
Option D: The noun 'strength' is neither concrete nor countable.
Hence options A, B and D are incorrect.