Answer:
The abbreviation that tells how the noun clause is used is SC - subject complement.
Explanation:
A noun clause, as the name shows, is a dependent clause that functions as a noun in a sentence. That means it can be everything a noun can be: subject, object of a verb or of a preposition, or a predicate nominative. Noun clauses begin with words such as what, whatever, when, which, how etc.
A subject complement is a predicative expression that follows a linking verb to describe the subject further. If it is a noun/pronoun, or a noun clause, it is called a predicate nominative.
Notice that in the sentence "A good night's sleep is what I need most,", the noun clause is "what I need most". It follows the linking verb "is" and provides further information about the subject "a good night's sleep". Thus, what we have here is a SC - subject complement.
