The correct answer is B.

Collateral mitigates counterparty credit risk but cannot eliminate it perfectly due to several structural and operational factors:

1. Thresholds: A non-zero threshold dictates that exposure below this amount remains uncollateralized, meaning the party is still exposed to some credit risk.
2. Minimum Transfer Amounts (MTAs) and Settlement Delays: MTAs create discrete gaps because collateral is only called when the exposure exceeds the MTA. Combined with settlement delays (the margin period of risk), these factors prevent collateral from perfectly tracking continuous movements in exposure.

Statements 2 and 4 are incorrect as collateral values can be accurately calculated based on marked-to-market valuations, and both collateral and exposure can be treated as continuous or discrete depending on the contract terms, but that is not the primary reason collateral fails to mitigate risk perfectly.