The primary factor that determines virtual warehouse cost in Snowflake is the length of time compute resources run (option D), as confirmed by Snowflake's official documentation and the unanimous community consensus. Snowflake charges for compute resources based on warehouse size and runtime measured in credits per second. While other factors like SQL statement types (A), number of tables/databases queried (B), and data storage amounts (C) can influence performance and indirectly affect runtime, they are not the direct cost drivers. The compute time is the fundamental billing metric, making D the correct primary factor.