Which solution will meet this requirement with the LEAST operational effort?

Explanations:

AWS Certificate Manager (ACM) certificates are used for securing communication between the Elastic Load Balancer and clients, but not between the ELB and EC2 instances. To establish end-to-end encryption, the EC2 instances also need to have their own certificates for SSL/TLS communication, which this option does not address.

Importing a third-party SSL certificate to ACM and installing it on the EC2 instances, while associating the imported certificate with the Elastic Load Balancer, enables encryption between the ELB and EC2 instances. This solution meets the requirement with minimal operational effort, as ACM simplifies the management of SSL certificates.

Deploying AWS CloudHSM for SSL certificate management is an unnecessary overhead for end-to-end encryption, and it introduces additional complexity compared to simply using ACM. CloudHSM is typically used for key management rather than managing SSL certificates for load balancers and EC2 instances.

Importing a third-party certificate bundle to ACM and installing it on EC2 instances adds unnecessary complexity. The ACM certificate could be used for the ELB, but managing a certificate on both the EC2 instances and ELB is more complicated than using an ACM-managed certificate for ELB and using EC2 certificates directly.