To optimize performance when writing to Cloud Spanner from a Go application, Google recommends using the Cloud Client Libraries (Option A). These libraries are purpose-built for Google Cloud services, provide idiomatic interfaces, and handle performance optimizations like connection management, retries, and batching.

• Option B (Google API Client Libraries) is not ideal, as these are older, auto-generated libraries with less intuitive interfaces and fewer optimizations for Cloud Spanner.
• Option C (custom gRPC client) and Option D (third-party HTTP client) are discouraged because they bypass Google's officially supported and tested libraries, potentially introducing inefficiencies or compatibility issues.