A. Enforce isolation and maintain a secure virtualization infrastructure. Cloud providers are responsible for ensuring that virtualized resources are isolated from each other, providing strong segregation between tenants. They must implement robust virtualization technologies and mechanisms to enforce this isolation, preventing unauthorized access or interference between different workloads. Additionally, cloud providers have the responsibility to maintain a secure virtualization infrastructure. This includes regularly patching and updating the underlying hypervisors, managing the host environment's security configurations, and implementing security measures to protect against vulnerabilities or attacks targeting the virtualization layer. While customers have their own security responsibilities within their virtual instances, the cloud provider's role primarily involves enforcing isolation and maintaining a secure virtualization infrastructure. Customers, on the other hand, are responsible for configuring the security settings within their virtual instances and monitoring and logging their own workloads (option B and E).

(Security Guidance p.93) Isolation ensures that compute processes or memory in one virtual machine/container should not be visible to another. It is how we separate different tenants, even when they are running processes on the same physical hardware. • The cloud provider is also responsible for securing the underlying infrastructure and the virtualization technology from external attack or internal misuse. This means using patched and up-to-date hypervisors that are properly configured and supported with processes to keep them up to date and secure over time. The inability to patch hypervisors across a cloud deployment could create a fundamentally insecure cloud when a new vulnerability in the technology is discovered.