The correct answer is A. Rinks
Step-by-step explanation:
In sentences, the subject is the word or words that indicate the agent performing the action of the sentence. In the case of the simple subject, this includes only the main word of the subject without any modifier such as adjectives that describe the subject. In the case of the sentence "Used for many sports and activities, indoor ice rinks require special care" the complete subject or agent is "indoor ice rinks"; however in this "indoor" and "ice" act as modifier of the noun "rinks" and therefore, the simple subject, in this case, is "rinks" as this is the main word in the subject.