Your organization has Compute Engine instances in us-east1, us-west2, and us-central1. Your organization also has an existing Cloud Interconnect physical connection in the East Coast of the United…

Configuring VPC routing in regional mode would restrict on-premises connectivity to only the subnets within the us-east1 region, where the Cloud Interconnect resides. This fails the requirement for high availability and access to 'all your other Virtual Private Cloud (VPC) subnets' in case of a regional outage.

This choice is a duplicate of C, hence also correct in principle, but C is the specified answer.

Configuring your VPC routing in global mode is the most cost-effective way to achieve high availability and ensure all VPC subnets across us-east1, us-west2, and us-central1 can access on-premises resources via a single Cloud Interconnect in us-east1. Global routing allows routes to be exchanged between all subnets in the VPC network and the on-premises network, preventing connectivity loss to on-premises if the us-east1 region (where the Interconnect is) experiences issues, as long as other regions are available.

Configuring VPC routing in regional mode would restrict on-premises connectivity to only the subnets within the us-east1 region, meaning instances in us-west2 and us-central1 would lose connectivity to on-premises if us-east1 or the Interconnect attachment has issues, failing the high availability requirement across regions.