Explanation

According to the AWS Shared Responsibility Model:

AWS is responsible for "Security OF the Cloud" which includes:

• Protecting the infrastructure that runs all AWS services
• Managing the host operating systems and virtualization layer
• Maintaining the configuration of infrastructure devices
• Maintaining the software that powers Availability Zones, data centers, and networking

The customer is responsible for "Security IN the Cloud" which includes:

• Management of guest operating systems (including updates and security patches)
• Application software and utilities installed by the customer
• Configuration of security groups and network access control lists
• Customer data and its classification
• Platform, applications, identity, and access management

Why Option A is correct:

• Guest operating systems refer to the operating systems running on EC2 instances or other compute resources that customers deploy
• Customers are responsible for patching, updating, and securing these operating systems

Why other options are incorrect:

• Option B: Maintenance of infrastructure device configuration is AWS's responsibility
• Option C: Management of host OS and virtualization is AWS's responsibility (this is the hypervisor layer)
• Option D: Maintenance of Availability Zone software is AWS's responsibility

This division of responsibilities allows AWS to focus on securing the cloud infrastructure while customers focus on securing what they put in the cloud.