The problem requires dynamic service discovery in a scalable way. Option B (Service Directory) is the correct choice because it provides a managed solution for service registration and discovery. By creating a Service Directory namespace and registering services during deployment, interservice communication can dynamically query the directory at runtime. This eliminates manual ENV variable maintenance.

• Option A involves querying Cloud Run/Cloud Functions APIs directly, which introduces latency, permission complexities, and lacks scalability.
• Option C relies on naming conventions, which are fragile and error-prone.
• Option D proposes Hashicorp Consul on a single VM, which is not scalable (single point of failure) and adds operational overhead. Service Directory (B) is the most scalable, managed, and integrated solution for Google Cloud environments.