Kubernetes supports two primary built-in modes of Service discovery for workloads: environment variables and DNS, making A correct.
Environment variables: When a Pod is created, kubelet can inject environment variables for Services that exist in the same namespace at the time the Pod starts. These variables include the Service host and port (for example, MY_SERVICE_HOST and MY_SERVICE_PORT). This approach is simple but has limitations: values are captured at Pod creation time and don’t automatically update if Services change, and it can become cluttered in namespaces with many Services.
DNS-based discovery: This is the most common and flexible method. Kubernetes cluster DNS (usually CoreDNS) provides names like service-name.namespace.svc.cluster.local. Clients resolve the name and connect to the Service, which then routes to backend Pods. DNS scales better, is dynamic with endpoint updates, supports headless Services for per-Pod discovery, and is the default pattern for microservice communication.
The other options are not Kubernetes service discovery modes. Labels and selectors are used internally to relate Services to Pods, but they are not what application code uses for discovery (apps typically don’t query selectors; they call DNS names). LDAP and RADIUS are identity/authentication protocols, not service discovery. DHCP is for IP assignment on networks, not for Kubernetes Service discovery.
Operationally, DNS is central: many applications assume name-based connectivity. If CoreDNS is misconfigured or overloaded, service-to-service calls may fail even if Pods and Services are otherwise healthy. Environment-variable discovery can still work for some legacy apps, but modern cloud-native practice strongly prefers DNS (and sometimes service meshes on top of it). The key exam concept is: Kubernetes provides service discovery via env vars and DNS.
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