Answer: Option D is the correct answer.
Explanation: The adjective clause is " which causes bumps on the skin".
Adjective clause is used to modify noun and pronoun in a sentence in order to give further description of the noun or pronoun. In this case "chicken pox" is the noun while the adjective clause gives more information about chicken pox by saying " which causes bumps on the skin" anyone who sees the clause will get a clear view of chicken pox.