The requirement is to release a new version of the application on Google Kubernetes Engine (GKE) with the ability to instantly roll back to the previous version if issues arise. This necessitates a deployment model that allows for immediate traffic switching without gradual transitions.

• Option A (Rolling deployment): This updates pods incrementally, replacing old instances with new ones. Rollback involves reversing the process step-by-step, which is not instant and can cause delays, especially with many pods.
• Option B (A/B testing): This routes a subset of traffic to the new version for comparison testing. It focuses on user experience analysis and does not inherently support instant rollback, as traffic adjustments are gradual.
• Option C (Canary deployment): This gradually shifts traffic from the old to the new version, allowing testing on a small scale. Rollback can be quick by reverting traffic, but it is not truly instant due to the progressive nature of traffic shifting.
• Option D (Blue/green deployment): This maintains two identical environments—'blue' (current version) and 'green' (new version). All traffic is switched at once to the new version. If issues occur, rollback is instantaneous by switching traffic back to the previous environment. This model is ideal for zero-downtime deployments and meets the 'instantly roll back' requirement.

For GKE, blue/green deployment can be implemented using Kubernetes services and ingress controllers to manage traffic routing, ensuring rapid fallback. The other options do not guarantee instant rollback, making D the best choice.