Explanation/Reference: https://cloud.google.com/compute/docs/instance-groups/creating-groups-of-managed-instances

The correct answer is C. Configure a target instance, and create a protocol forwarding rule for each external IP address to be mapped to the instance.
This answer is based on the following facts:
A target instance is a Compute Engine instance that handles traffic from one or more forwarding rules1. You can use target instances to forward traffic to a single VM instance from one or more external IP addresses2.
A protocol forwarding rule specifies the IP protocol and port range for the traffic that you want to forward3. You can use protocol forwarding rules to forward traffic of any IP protocol, including ESP4.
The other options are not correct because:
Option A is not possible. You cannot create protocol forwarding rules for a target pool. A target pool is a group of instances that receives traffic from a network load balancer5.
Option B is not suitable. You do not need to create an external network load balancer for each external IP address. An external network load balancer distributes traffic among multiple backend instances based on the destination IP address and port. You can use a single load balancer with multiple forwarding rules to map multiple external IP addresses to the same backend service.
Option D is not feasible. You cannot add multiple external IP addresses to a single network interface of a Compute Engine instance. Each network interface can have only one external IP address that is either ephemeral or static. You can use alias IP ranges to assign multiple internal IP addresses to a single network interface, but not external IP addresses.