Final answer:
The clearinghouse undertakes the opposite side of every futures trade, providing a guarantee of settlement and managing the process of marking to market.
Step-by-step explanation:
The entity that undertakes the opposite side of every futures trade is d) The clearinghouse. A clearinghouse acts as the counterparty to both sides of a futures contract, ensuring that if one party defaults, the other party will still have the contract upheld. This system provides a guarantee that the trade will be settled as agreed, thereby reducing the credit risk for traders. The clearinghouse also manages the daily settling of profits and losses, known as marking to market.