✅ The word "ancient" directly describes the noun "scroll," indicating its age or historical characteristic, thus functioning as a descriptive adjective.

❌ "Discovered" is a past participle that is part of an adjectival participial phrase ("discovered in a hidden chamber") modifying "scroll," but it is not the single adjective directly modifying it in the way "ancient" does.

❌ "Hidden" is an adjective, but it modifies the noun "chamber," describing the type of chamber, not the scroll.

❌ "Contained" is the main verb of the sentence, indicating an action performed by the scroll, not a descriptive quality of the scroll itself.

✅ The clause "who was very impressed with the design" begins with the relative pronoun "who," which refers back to and describes the noun "client." This structure is characteristic of an adjective (or relative) clause, as its primary function is to modify a noun or pronoun by providing additional descriptive information.

❌ Option A is incorrect because adverb clauses typically modify verbs, adjectives, or other adverbs, indicating 'when,' 'where,' 'how,' 'why,' or 'under what condition' an action occurs, and they do not typically begin with "who" to describe a noun.

❌ Option C is incorrect because noun clauses function as nouns (e.g., subject, object, complement) within a sentence; this clause is clearly providing descriptive information about "client," not acting as a noun itself.

❌ Option D is incorrect because an independent clause can stand alone as a complete sentence; "who was very impressed with the design" cannot stand alone as it starts with a relative pronoun that links it to the main clause, making it a dependent clause.