A bastion host is a hardened system placed at a network boundary to absorb attacks and limit exposure. When deployed to mitigate risks from zero-day vulnerabilities, it acts as a compensating control. CompTIA Security+ SY0-701 defines compensating controls as alternative safeguards used when primary controls are insufficient or unavailable-such as when no patch exists for a zero-day.
Detective controls (B) identify issues but do not reduce exposure. Operational controls (C) refer to procedural or human-driven processes. Physical controls (D) secure physical environments (e.g., locks, cameras).
Because a bastion host compensates for the lack of a patch, the correct answer is A: Compensating.