The two new VPC CIDRs, 10.0.32.0/21 and 10.0.40.0/21, are contiguous and can be summarized into a single supernet: 10.0.32.0/20. To advertise routes from AWS to an on-premises network via a Direct Connect Gateway (DXGW), the prefixes must be added to the DXGW's "allowed prefixes" list. Since there is only one entry remaining before the prefix list quota is reached, adding the single summary route 10.0.32.0/20 is the only solution that meets all requirements. This allows the DXGW to advertise the summarized route over BGP to the on-premises location, providing connectivity to both new VPCs while consuming only one entry in the list.

A: This requires two entries, violating the quota, and incorrectly refers to "AWS managed prefix lists" instead of the DXGW's allowed prefixes.

B: This requires two entries, which violates the one-entry-remaining quota for the prefix list.

C: This incorrectly refers to "AWS managed prefix lists." The correct mechanism is the Direct Connect Gateway's allowed prefixes list.

1. AWS Documentation, Direct Connect User Guide: "Working with Direct Connect gateways - Allowed prefixes". This section states, "When you specify an allowed prefix list on a Direct Connect gateway, only routes that match the prefixes are advertised over the Border Gateway Protocol (BGP) sessions for the attached transit virtual interfaces... You can summarize the prefixes from the attached VPCs and VPNs." This confirms that using a summarized prefix in the allowed prefixes list is the correct method.

2. AWS Documentation, Direct Connect User Guide: "AWS Direct Connect quotas". This document lists the "Prefixes advertised from a Direct Connect gateway to an on-premises network per BGP session" quota, which is 100 by default. The question's scenario of reaching this quota limit necessitates a strategy like route summarization.

3. AWS Documentation, AWS Transit Gateway User Guide: "Route evaluation order". This document explains how routes are evaluated, but for hybrid connectivity with Direct Connect, the advertisement to on-premises is ultimately controlled by the Direct Connect Gateway's configuration, specifically the allowed prefixes. The TGW propagates its routes to the DXGW, which then filters them.