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.

Comprehensive and Detailed In Depth Explanation:
Let's analyze each option to find the one that meets the requirements of no startup time for new traffic, two idle instances, and minimal administrative overhead:
A). Unchecking "Serve this revision immediately" and using a traffic simulation tool: Unchecking "Serve this revision immediately" does prevent the new revision from receiving traffic immediately. However, manually using a traffic simulation tool adds administrative overhead. It also doesn't guarantee that two idle instances will be ready before traffic is shifted; you would need to monitor and adjust traffic manually based on the simulation.
B). Configuring service autoscaling and setting the minimum number of instances to 2: Service-level autoscaling applies to all revisions of the service. Setting the minimum instances at the service level would ensure at least two instances are running across all active revisions, not specifically for the new revision before traffic shift.
C). Configuring revision autoscaling for the new revision and setting the minimum number of instances to 2:
This is the correct approach. By configuring revision autoscaling specifically for the new revision and setting the minimum number of instances to 2, Cloud Run will ensure that at least two instances of the new version are running and ready to serve traffic before you redirect any traffic to it. This eliminates startup latency when you do shift traffic. It also minimizes administrative overhead as Cloud Run manages the instance scaling based on this configuration.
D). Configuring revision autoscaling for the existing revision and setting the minimum number of instances to
2: This would ensure the existing version has at least two idle instances, which doesn't directly address the requirement of having idle instances ready for the new version before traffic redirection.
Google Cloud Documentation References:
Cloud Run Autoscaling: https://cloud.google.com/run/docs/configuring/min-instances - This document explains how to configure minimum and maximum instances for Cloud Run services and revisions. It clarifies that you can set minimum instances at the revision level to ensure instances are always ready.
Cloud Run Traffic Management: https://cloud.google.com/run/docs/managing/traffic - This describes how to deploy new revisions and gradually shift traffic between them. Combining minimum instances on the new revision with traffic splitting allows for zero-downtime deployments with pre-warmed instances.