As far as i am concerned, the correct option is option "d". "Mushrooms, which come in many varieties, can be expensive is a sentence that contains an adjective clause. Any adjective clause needs to completely meet three important and basic requirements.Firstly the sentence should have a verb and a subject in it to be an adjective clause. Secondly it actually begins with relative pronouns like who, whom, whose etc. In regards to this sentence, the adjective clause is following the word that it is describing. Last but not the least important is the fact that the sentence should be like answering a question starting with who, what etc.
