The correct answer is A. Residual risk.

Residual risk is the risk that remains after safeguards, controls, or mitigation strategies have been applied. In some cases, a risk cannot be fully eliminated or mitigated. The organization must then decide whether to accept, transfer, avoid, or further manage the remaining risk.

This aligns with CompTIA Security+ SY0-701 risk management concepts, especially risk treatment, risk acceptance, risk tolerance, and residual risk.

Why the other options are incorrect:

B. Risk appetite

Risk appetite is the amount and type of risk an organization is willing to accept in pursuit of its objectives. It does not specifically describe a risk that remains after mitigation attempts.

C. Reviewed risk

This is not the best formal risk management term for a risk that cannot be mitigated.

D. Risk register

A risk register is a document or system used to track identified risks, risk owners, likelihood, impact, treatment plans, and status. It does not describe the remaining risk itself.

Therefore, a risk that cannot be fully mitigated is best described as residual risk.