The correct answer is Enabling and managing database encryption settings. This responsibility falls to the customer for Amazon RDS for PostgreSQL.

AWS manages the underlying infrastructure the host operating system and the database engine as part of the managed RDS service and customers retain control over their data and configuration choices. Enabling encryption at rest and managing associated keys and key policies is a customer side activity because it directly affects how customer data is protected and who can access the encryption keys.

Patching the operating system that supports the DB instance is handled by AWS because RDS abstracts the host operating system and AWS applies OS level maintenance for managed instances.

Managing the physical servers that host Amazon RDS is the responsibility of AWS since the cloud provider owns and operates the data center hardware facilities and network infrastructure.

Applying database engine patches to the managed RDS instance is performed by AWS for RDS while customers can influence timing by setting maintenance windows but do not directly apply engine patches themselves.

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