Final answer:
The sentence 'While he sat sadly in the dentist's chair, the boy reluctantly opened his mouth' contains two clauses: a dependent clause and an independent clause.
Step-by-step explanation:
The sentence in question has two clauses. A clause is a part of a sentence that contains both a subject and a verb. In the sentence provided, there are two distinct parts that each contain a subject and a verb. The first clause is 'While he sat sadly in the dentist's chair,' which is a dependent clause because it cannot stand alone as a complete sentence. The second clause is 'the boy reluctantly opened his mouth,' which is an independent clause as it can stand alone as a complete sentence.
Therefore, the correct answer is B) 2 clauses.