Final answer:
The sentence in question includes an adjective clause, which provides additional description to the noun 'dog'. The correct answer is option b.
Step-by-step explanation:
The sentence "The dog who has long hair is really friendly" contains an adjective clause. This clause is "who has long hair" and it describes the noun "dog", indicating which one of the dogs we're talking about.
Adjective clauses function as adjectives, hence the name, because they describe or give more information about a noun or pronoun in the sentence.
An adverb clause, on the other hand, would modify a verb, an adjective, or another adverb, indicating when, where, why, or how something happened. For example, in the sentence "The dog ran quickly because it was scared," the clause "because it was scared" is an adverb clause explaining why the dog ran quickly.
The correct option here is 'b) Adjective'.