The correct choice is Adopt a blue green deployment and switch traffic between environments.

Adopt a blue green deployment and switch traffic between environments works because you deploy the new version into a separate but identical environment and then shift user traffic from the old environment to the new one through the load balancer or service router. This approach minimizes or eliminates user-visible interruption because the new environment is fully warmed and health checked before any traffic is routed to it. It also provides an immediate and safe rollback path by switching traffic back to the previous environment if a problem is detected.

Adopt a blue green deployment and switch traffic between environments is especially useful for migrating a monolith because you can deploy a complete copy of the application, validate it under real traffic patterns, and then cut over without in place upgrades that risk disrupting sessions or compatibility.

Use a rolling update of application pods is not the best answer because rolling updates replace pods in place and can still cause interruptions for stateful or sessionful monoliths and they do not provide the same instant cutover and rollback separation that blue green provides.

Use Cloud Run traffic splitting to shift requests gradually is platform specific and is about shifting traffic between revisions in Cloud Run. It can be useful for controlled rollouts, but it is not a general blue green cutover method and may not suit a legacy monolith without refactoring or moving to that specific platform.

Scale the monolith and replace components with microservices over time is a migration strategy rather than a deployment technique for zero downtime. Scaling and gradual refactoring help long term, but they do not by themselves guarantee interruption free deployments during a release.

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