The correct answer is D because it meets the following requirements:
It matches the hub-and-spoke model of the on-premises network, where each spoke is a separate VPC network that is connected to a central hub VPC network.
It minimizes management overhead and cost, because VPC Network Peering is a simple and low-cost way to connect VPC networks without using any external IP addresses or VPN gateways1.
It uses default networking quotas and limits, because VPC Network Peering does not consume any quota or limit for VPN tunnels, external IP addresses, or forwarding rules2.
It prevents connectivity between the spokes, because VPC Network Peering is non-transitive by default, meaning that a spoke can only communicate with the hub, not with other spokes1. To enforce this restriction, a third-party network appliance can be used as a default gateway in each spoke VPC network, which can filter out any traffic destined for other spokes3.
Option A is incorrect because it does not minimize cost, as Cloud VPN charges for egress traffic and requires external IP addresses for the VPN gateways4. Option B is incorrect because it does not prevent connectivity between the spokes, as VPC Network Peering allows direct communication between peered VPC networks by default1. Option C is incorrect because it does not minimize cost or use default quotas and limits, for the same reasons as option A.
Reference:
VPC Network Peering overview | VPC
Quotas and limits | VPC
Hub-and-spoke network architecture | Cloud Architecture Center
Cloud VPN overview | Google Cloud