The question focuses on securing communication between microservices using a managed service. Option D (Anthos Service Mesh) is correct because Anthos is Google's fully managed service mesh that provides mTLS for secure inter-service communication. Option B involves deploying open-source Istio, which is not a managed service. Options A and C address SSL certificates for external traffic or certificate management but do not provide a managed service mesh solution for internal service-to-service security.