The most effective method for reducing overall counterparty credit exposure to "almost zero" is through the implementation of strict collateral agreements, ensuring sufficient collateral (both Initial Margin and Variation Margin) is systematically posted. Fully collateralizing the exposure directly mitigates the potential loss in the event of default to a negligible amount (leaving only gap risk during the margin period of risk).

• Option B is impractical and ineffective here because the market is dominated by a "handful of other firms," making proper diversification impossible.
• Option C (netting) reduces gross exposure to net exposure but typically does not reduce it to "almost zero" unless the outstanding trades happen to perfectly offset one another.
• Option D (credit derivatives) merely substitutes the counterparty risk of the original trading partners with the counterparty risk of the protection seller. Overall counterparty risk is not eliminated.