Answer: Patch the guest operating system of Amazon EC2 instances.

## Explanation According to the AWS Shared Responsibility Model: - **AWS is responsible for "Security OF the Cloud"** - This includes: - Protecting and securing the AWS global infrastructure - Maintaining the physical hardware - Securing the underlying virtualization infrastructure - Patching and maintaining AWS managed services like Amazon RDS - **Customers are responsible for "Security IN the Cloud"** - This includes: - Patching the guest operating system of EC2 instances - Managing security groups and network access control lists - Managing IAM users and permissions - Encrypting data at rest and in transit - Application-level security Therefore, patching the guest operating system of Amazon EC2 instances is clearly a customer responsibility, while the other options (hardware security, AWS global infrastructure protection, and RDS patching) are AWS responsibilities.

Author: Ritesh Yadav