To ensure only images successfully built by your trusted CI/CD pipeline are deployed to production on GKE clusters, the most effective measure is to set up the Kubernetes Engine clusters with Binary Authorization (Option D). Binary Authorization is a deploy-time security control that ensures only trusted container images are deployed on Google Kubernetes Engine. It allows you to require images to be signed by trusted authorities during the development process and enforces this requirement at deployment time. Enabling Cloud Security Scanner (Option A) is more about identifying security vulnerabilities in your App Engine, Compute Engine, and Google Kubernetes Engine applications. Enabling Vulnerability Analysis on the Container Registry (Option B) helps in identifying vulnerabilities in your container images but does not enforce deployment policies. Setting up the Kubernetes Engine clusters as private clusters (Option C) restricts access to the Kubernetes API server to only internal IP addresses but does not control which images can be deployed.