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How are option contracts enforced?
By their agreements with the central counterparty - in the US, the exchange or the Options Clearing Corporation, which interposes itself between the counterparties of each trade and guarantees that they settle. From the CCP article: A clearing house stands between two clearing firms (also known as member firms or participants). Its purpose is to reduce the risk a member firm failing to honor its trade settlement obligations. A CCP reduces the settlement risks by netting offsetting transactions between multiple counterparties, by requiring collateral deposits (also called "margin deposits"), by providing independent valuation of trades and collateral, by monitoring the credit worthiness of the member firms, and in many cases, by providing a guarantee fund that can be used to cover losses that exceed a defaulting member's collateral on deposit. Exercisers on most contracts are matched against random writers during the assignment process, and if the writer doesn't deliver/buy the stock, the OCC does so using its funds and goes after the defaulting party.