A risk register is a document that lists potential risks, along with their severity, impact, and the actions that should be taken to mitigate them. It is used to systematically identify and manage risks. The correct answer, 'Documenting identified risks and their mitigation plans,' succinctly describes the fundamental purpose of a risk register. The incorrect options, while related to risk management, do not directly correlate with the primary function of a risk register. Conducting routine audits is related to the assessment and review of existing security measures, not directly to the risk register. Implementing security policies is about enforcing rules and practices to secure cloud environments. Creating a business continuity plan is related to preparing for and responding to disruptive events, which may utilize information from a risk register, but is not its main purpose.