Replace the underlined word with its noun form.
The dark room frightened me.
A. The darkness of the room frightened me.
B. The room was dark and frightened me.
C. I was frightened by the darkness of the room.
D. None of the above.

Hint:At first, we have to see the parts of speeches for each and every word in the given sentence. ‘The’ is an article which signifies or specifies the noun ‘room’. The word ‘dark’ is an adjective which shows the quality or state of the noun ‘room’. The word ‘frightened’ is a verb and this is in past form. And, the word ‘me’ is a pronoun and this is set at the end of the sentence according to the ‘Subject-Verb Agreement’..

Complete answer:
In the given question, we have been asked to convert the adjective ‘dark’ to its noun form.
 In order to convert an adjective to a noun, the suffixes like -ness (happiness) and -ity (divinity) are generally added. In the given sentence, the underlined word ‘dark’ is to be converted. The noun form of this adjective is ‘darkness’.

A. In the first option, the adjective has been converted into its noun form correctly. And, most importantly, its form of active voice has not been altered.

B. In the second option, the word ‘dark’ has not been converted into its ‘noun’ form. Just we can say here the sentence has been reframed a bit without converting its existing form of parts of speech.

C. Although in the third option, we can see that the word ‘dark’ word has been converted into its noun form. But it is not correct because the original sentence has been converted into passive which is not asked in the original question.

Hence the correct answer is option ‘A’.

Note:We have seen in this question that while converting an adjective into a noun, suffixes are usually added. But there are so many other ways by which we can convert the noun to adjective and vice versa. Specifically, as we have iterated before, we need to focus the quality of adjectives in this regard to find out how they are qualifying the nouns.