Which combination of steps will meet these requirements MOST cost-effectively?

Explanations:

Establishing an AWS Site-to-Site VPN connection to each VPC is not cost-effective for connecting multiple VPCs, as it would require multiple VPN connections and could lead to increased management overhead and costs. Moreover, VPN connections may not consistently meet the single-digit latency requirements.

Associating an AWS Direct Connect gateway with the transit gateway that is attached to the VPCs allows for a scalable and cost-effective way to manage network traffic. Direct Connect provides low-latency connectivity and can connect multiple VPCs through the transit gateway, meeting the latency and cost requirements effectively.

Establishing a Site-to-Site VPN connection to an AWS Direct Connect gateway does not directly address the need to connect multiple VPCs cost-effectively. While it provides secure connectivity, it may not be the most efficient method for achieving single-digit latencies across workloads in both AWS and on-premises environments.

Establishing an AWS Direct Connect connection and creating a transit virtual interface (VIF) to a Direct Connect gateway allows for high-speed, low-latency connections. This setup enables the hybrid architecture to communicate efficiently with both on-premises data centers and AWS workloads, supporting the requirement for single-digit latencies in a cost-effective manner.

Associating AWS Site-to-Site VPN connections with the transit gateway that is attached to the VPCs would not provide the low-latency connectivity required for hybrid architecture. While it allows for some level of connectivity, it is not the most efficient solution for multiple VPCs compared to using Direct Connect.