To ensure minimal disruption to the existing architecture during migration from Microsoft SQL Server to Google Cloud, the best option is to migrate to Cloud SQL for SQL Server (Option A). This service is specifically designed to support SQL Server databases in the cloud, offering compatibility with SQL Server's features and tools, including SSAS, SSRS, and SSIS, which are critical for your low-latency reporting application. Migrating to Cloud SQL for PostgreSQL (Option B) would require significant changes to your database schema and application logic due to differences between SQL Server and PostgreSQL. Migrating to Compute Engine (Option C) or Google Kubernetes Engine (Option D) would involve managing the SQL Server instances yourself, which could introduce complexity and potential disruptions, contrary to the goal of minimal disruption.