Protect is correct.

The Protect function concentrates on designing and applying safeguards so that essential infrastructure services remain available. It encompasses access control, data security, protective technologies, and maintenance practices that reduce the likelihood of disruptions and preserve availability.

Detect is incorrect because it is focused on identifying anomalies and security events rather than on implementing safeguards to maintain service availability.

Respond is incorrect because it addresses actions taken after an incident to contain and mitigate impact rather than proactively designing protections to keep services available.

Identify is incorrect because it deals with understanding assets, risks, and governance and not directly with applying safeguards to ensure availability.

Full AWS Practitioner Certification Question