Option C is a valid solution that involves ordering a new Dedicated Interconnect for a colocation facility closest to us-east4 and using VPC global routing to access workloads in us-central1. This would reduce data transfer costs between us-east4 and the on-premises location, but it would still require data transfer between us-central1 and us-east4 over the Google Cloud network, which may result in additional data transfer costs.
Option B, on the other hand, involves keeping the existing Dedicated Interconnect and deploying VLAN attachments to Cloud Routers in both us-east4 and us-central1. This would minimize data transfer costs by keeping traffic between the two regions on the on-premises network and not sending it over the Google Cloud network. It also leverages the existing connection to reduce costs.
Therefore, option B is a better solution to minimize data transfer costs when deploying VLAN attachments in this scenario.