The correct answer to the question is A: Ensuring that sufficient collateral is posted by counterparties. This method is the most effective in mitigating the overall counterparty exposure because it provides a direct safeguard against the risk of default by the counterparty. By holding high-quality collateral, such as cash or short-term investment grade government bonds, the company can ensure that it has a form of security to cover potential losses if the counterparty fails to fulfill its obligations under the derivative contracts.

The other options are not as effective for the following reasons:

• Option B: Diversifying among counterparties is not a viable solution in this scenario because the company already operates in a market dominated by a few firms, limiting the possibility for further diversification.
• Option C: Cross-product netting on a single counterparty basis would only reduce exposure to one counterparty and does not address the overall counterparty risk across all contracts.
• Option D: Purchasing credit derivatives, such as credit default swaps, would not reduce the counterparty risk but rather shift it from the individual counterparties to the institution that wrote the CDS, thus not achieving the goal of reducing the overall exposure to almost zero.

The explanation is grounded in the principles of credit risk management, where collateral serves as a critical tool for mitigating counterparty risk. The reference to Jon Gregory's "The xVA Challenge: Counterparty Credit Risk, Funding, Collateral, and Capital" provides further insight into the strategies and mechanisms for managing and mitigating counterparty risk, including the role of collateral agreements and credit support annexes (CSAs) within the ISDA Master Agreement.