The most suitable solution for the company’s requirements is to enable Default Host Configuration Management in Systems Manager to manage the EC2 instances. This solution will allow the company to patch the EC2 instances without disrupting the running applications and without manually creating or modifying IAM roles or users.

Default Host Configuration Management is a feature of AWS Systems Manager that enables Systems Manager to manage EC2 instances automatically as managed instances. A managed instance is an EC2 instance that is configured for use with Systems Manager. The benefits of managing instances with Systems Manager include the following:

Connect to EC2 instances securely using Session Manager.

Perform automated patch scans using Patch Manager.

View detailed information about instances using Systems Manager Inventory.

Track and manage instances using Fleet Manager.

Keep SSM Agent up to date automatically.

Default Host Configuration Management makes it possible to manage EC2 instances without having to manually create an IAM instance profile. Instead, Default Host Configuration Management creates and applies a default IAM role to ensure that Systems Manager has permissions to manage all instances in the Region and account where it is activated.If thepermissions provided are not sufficient for the use case, the default IAM role can be modified or replaced with a custom role1.

The other options are not correct because they either have more operational overhead or do not meet the requirements. Creating a new IAM role, attaching the AmazonSSMManagedInstanceCore policy to the new IAM role, and attaching the new IAM role and the existing IAM role to the EC2 instances is not correct because this solution requires manual creation and management of IAM roles, which adds complexity and cost to the solution.The AmazonSSMManagedInstanceCore policy is a managed policy that grants permissions for Systems Manager core functionality2. Creating an IAM user, attaching the AmazonSSMManagedInstanceCore policy to the IAM user, and configuring Systems Manager to use the IAM user to manage the EC2 instances is not correct because this solution requires manual creation and management of IAM users, which adds complexity and cost to the solution.An IAM user is an identity within an AWS account that has specific permissions for a single person or application3. Removing the existing policies from the existing IAM role and adding the AmazonSSMManagedInstanceCore policy to the existing IAM role is not correct because this solution may disrupt the running applications that rely on the existing policies for accessing RDS databases.An IAM role is an identity within an AWS account that has specific permissions for a service or entity4.