The goal is to ensure high availability across regions in a cost-effective manner. The existing setup has a single VLAN attachment in us-east1. To handle region failures, the solution must provide redundant interconnects in different regions.

1. Global Routing Mode: Configuring the VPC in global routing mode ensures that routes propagate across all regions, allowing traffic to use any available interconnect regardless of region.
2. Redundant Interconnect in Another Region: Adding a VLAN attachment in us-west2 (a different region) ensures that if us-east1 goes down, the interconnect in us-west2 maintains connectivity. This provides geographic redundancy, which is critical for region-level failures.

Option C is correct because it combines global routing (enabling cross-region route propagation) with a new interconnect in us-west2, ensuring connectivity even if us-east1 fails. Other options either rely on redundant interconnects in the same region (which do not address region failures) or use regional routing (which limits route propagation). While adding a new interconnect in us-west2 may incur additional costs, it is necessary to meet the HA requirement for region failures in the most cost-effective way possible under the constraints.