The most efficient solution with minimal changes is to establish a Transit Gateway (TGW) peering connection. TGW peering is a feature designed specifically to connect two Transit Gateways, enabling routing between their respective networks. Since both the company's TGW (TGW-C) and the partner's TGW (TGW-P) are in the same AWS Region (us-east-1), an intra-region peering attachment can be created. This approach allows both organizations to maintain their independent network architectures and routing domains while enabling secure, high-bandwidth connectivity. The process involves creating a peering request from one TGW, accepting it from the other, and then updating the TGW route tables to direct traffic over the peering connection.

A. This describes a complex "transit VPC" pattern. It requires deploying and managing a third-party router, which is significantly more complex and costly than using the native TGW peering feature.

B. An IPsec VPN is a viable but suboptimal solution. It introduces encryption overhead and typically has lower bandwidth limits compared to a TGW peering connection, which is the purpose-built, higher-performance option for this scenario.

D. This would force the partner to abandon their own Transit Gateway (TGW-P) and attach their VPCs to the company's TGW. This constitutes a major architectural change for the partner, violating the "minimum changes" requirement.

1. AWS Transit Gateway User Guide - Peering attachments: "You can create a peering attachment between your transit gateways in the same AWS Region... A transit gateway peering attachment enables you to route traffic between the transit gateway route tables." This document explicitly describes the functionality used in the correct answer.

Source: AWS Documentation, Transit Gateway User Guide, "Transit gateway peering attachments".

2. AWS Transit Gateway User Guide - How transit gateways work: This guide explains that Transit Gateway acts as a cloud router to simplify network architectures. The peering feature is an extension of this simplification for inter-TGW communication, making complex workarounds like a router in a separate VPC (Option A) unnecessary.

Source: AWS Documentation, Transit Gateway User Guide, "How transit gateways work".

3. AWS Transit Gateway User Guide - Example: Peered transit gateways: "The owner of TGW A creates a peering attachment... The owner of TGW B accepts the peering attachment request... The owners update their transit gateway route tables to send traffic to the peering attachment." This section details the exact steps required for the solution in option C.

Source: AWS Documentation, Transit Gateway User Guide, "Example: Peered transit gateways".

4. AWS Resource Access Manager (RAM) User Guide - Shareable AWS resources: This document confirms that a Transit Gateway can be shared (as in Option D). However, it clarifies that sharing allows another account to create attachments to the shared TGW, not merge two TGWs. This confirms the mechanism for Option D but highlights why it would require a significant change to the partner's network.

Source: AWS Documentation, AWS RAM User Guide, "Shareable AWS resources".