Which solution will meet these requirements?

Explanations:

Configuring the EC2 instances to be part of a cluster placement group allows them to be physically located close together within the same Availability Zone. This provides low-latency network performance and high network throughput, making it ideal for HPC workloads that require tightly coupled communication between nodes.

Launching EC2 instances with Dedicated Instance tenancy means the instances run on hardware dedicated to a single customer. While this can enhance isolation, it does not specifically optimize for low-latency or high throughput needed for HPC workloads, and may not improve inter-node communication performance.

Launching EC2 instances as Spot Instances offers cost savings but does not guarantee the performance required for low-latency and high-throughput communication. Spot Instances can be interrupted, which can disrupt tightly coupled HPC workloads.

Configuring an On-Demand Capacity Reservation ensures that capacity is reserved for the instances but does not inherently improve network performance characteristics like latency and throughput. It does not address the requirement for tightly coupled communication among nodes.