You created a new instance template with a non‑critical update for a managed instance group but must not modify running instances; new instances must include the update. What action do you take?

The correct option is to start a new rolling update using Opportunistic update mode. This mode applies the new instance template only to new instances, or instances that are restarted or re-created for other reasons (e.g., autohealing, zone rebalancing). This perfectly matches the requirement to not modify running instances while ensuring new instances get the update. A rolling restart operation would restart all instances, applying the new template to all of them, which violates the requirement to not modify running instances. A rolling replace operation also replaces all instances, which is similar to a restart in this context and also violates the requirement. A rolling update using Proactive update mode would immediately begin updating existing instances to the new template, which is contrary to the requirement that running instances must not be modified.