An abstract noun is a noun that cannot be seen or touched. It is an idea, quality or state.
Option A: From
the options, 'bitterness' refers to a quality (formed by the
adjective-bitter), hence it's an abstract noun and option A is correct.
Option B: 'Lemon
curd' is a dessert spread used as a filling in tarts and pies. It is a
concrete, compound noun and isn't abstract. Hence option B is incorrect.
Option C: 'Rind'
is a noun. It refers to the peel of the lemon. It isn't a state or
a condition. It's a concrete noun, hence option C is incorrect.
Option D is incorrect because the answer lies in A.