Why B is correct (based on official Google Cloud documentation: https://cloud.google.com/network-connectivity/docs/vpn/concepts/choosing-networks-routing#ts-ip-ranges)

The scenario specifies:

• On-premises legacy VPN device that only supports IKEv1 and cannot use BGP.
• A Classic VPN gateway is already configured.
• Need for a policy-based VPN tunnel.

According to the Google Cloud documentation on traffic selectors:

“When you use IKEv1, you can only supply a single CIDR for the local traffic selector and a single CIDR for the remote traffic selector. … Cloud VPN does not support creating a VPN tunnel by using IKEv1 with multiple Child SAs, each with a single CIDR.”

Because of this strict IKEv1 limitation (single Child SA per tunnel), you cannot specify multiple individual /24 subnets in the traffic selectors (as option C attempts). Option C would only work with IKEv2.

Recommended approach for multiple subnets with IKEv1:

• Use a single, broader supernet (/22 in this case) that summarizes (covers) all the required subnets on each side.
• On-premises subnets: 172.16.100.0/24 + 172.16.101.0/24 + 172.16.102.0/24 → summarized as 172.16.100.0/22
• Google Cloud subnets: 192.168.100.0/24 + 192.168.101.0/24 + 192.168.102.0/24 → summarized as 192.168.100.0/22

Traffic selector configuration (from Cloud VPN perspective):

• LOCAL_TS (GCP side) = 192.168.100.0/22
• REMOTE_TS (on-premises side) = 172.16.100.0/22

This matches option B exactly.

On the on-premises legacy device, you must configure the reversed selectors (local = on-prem 172.16.100.0/22, remote = GCP 192.168.100.0/22) to match.

Additional important points for the exam

• After creating the tunnel with these broad selectors, Google Cloud automatically creates static routes in the VPC for the remote range (172.16.100.0/22) pointing to the tunnel (when using the console).
• You may need to create more specific static routes if you want finer control, but the traffic selector itself must be the single summarized CIDR for IKEv1 compatibility.
• The /22 summarization works cleanly here because the three /24s are contiguous and align properly under 172.16.100.0/22 and 192.168.100.0/22.
• Option A has the LOCAL_TS and REMOTE_TS swapped — that is incorrect.
• Option C tries to list multiple /24s — not supported with IKEv1 on Cloud VPN.
• Option D uses 0.0.0.0/0 for remote — this turns it into route-based behavior and is not a proper policy-based setup for specific subnets.

Best practices / alternatives (for real-world or exam context)

• If possible, upgrade the on-premises device to support IKEv2 → then option C (multiple CIDRs) becomes valid.
• Prefer HA VPN with BGP (dynamic routing) whenever the on-premises side can support it — it uses 0.0.0.0/0 selectors and avoids these limitations entirely.
• For legacy IKEv1-only devices: summarization (supernetting) + policy-based static routing is the standard workaround.

This question tests your understanding of IKEv1 traffic selector limitations in Classic VPN policy-based tunnels. It's a common exam topic.