The correct answer is B. The default of a clearing member can indeed lead to a loss of liquidity for the CCP. When a clearing member defaults, the CCP is obligated to fulfill the defaulted member's obligations to the other clearing members. To do this, the CCP typically uses its own resources, such as its capital, guaranty funds, and default funds, to make the required payments. However, in the event of a large or unexpected default, these resources may not be sufficient to cover the losses. This insufficiency can lead to a loss of liquidity for the CCP, as it may struggle to meet its financial obligations. This scenario illustrates the exposure of CCPs to liquidity risk, despite the various risk mitigation measures they may have in place.

Choice A is incorrect. While it's true that CCPs require their clearing members to post initial and variation margins, this does not completely eliminate their exposure to liquidity risk. For instance, if a clearing member defaults and the value of the posted collateral falls significantly, the CCP could face a liquidity shortfall.

Choice C is incorrect. Even with properly calculated margin requirements, CCPs can still be exposed to liquidity risk due to various factors such as market volatility or default of multiple members which may lead to significant losses exceeding the available margins.

Choice D is incorrect. Liquidity risk for CCPs is not solely related to operational risk. It also arises from other sources such as credit risk (default by a member), market risk (changes in value of collateral), and even systemic risks (failure of financial institutions or disruption in financial markets).