The root cause of the failed deployment is a missing environment variable, which is a configuration issue. To prevent this issue from recurring, the best approach is to implement static code analysis in the CI pipeline (Option B). Static code analysis can help identify missing or incorrect environment variables before the code is deployed. While canary deployments (Option A) can help mitigate the impact of failed deployments, they do not address the root cause. Integration tests (Option C) are important but may not catch configuration issues like missing environment variables. Enabling Cloud Audit Logs (Option D) is useful for monitoring and auditing but does not prevent configuration errors. Therefore, the correct answer is B.