Option A is the correct answer because ‘curiosity’ means ‘a
strong desire to know something’. Thus, ’ curiosity’ is the most appropriate
noun to complete the meaning of this sentence.
Option B is incorrect because ‘curious’ is an adjective and it is grammatically
incorrect for this sentence.
Option C is incorrect because although ‘curiousness’ is a noun it is not the
grammatically appropriate word for this sentence.
Option D is incorrect because ‘curiously’ is an adverb and it is grammatically
incorrect for this sentence.