Correct option is D
The underlined clause is an essential part of the sentence structure and can stand on its own.
Information Booster
A principal clause (main clause) expresses the central idea of a sentence and does not depend on another clause.
Additional Knowledge
Noun, adjective, and adverb clauses function differently: noun clauses act as subject or object; adjective clauses describe nouns; adverb clauses modify verbs, adjectives, or other adverbs.