What is the PRIMARY factor that determines the cost of using a virtual warehouse in...

The primary factor that determines the cost of using a virtual warehouse in Snowflake is D. The length of time the compute resources in each cluster run. Snowflake's pricing model for compute usage is based on the concept of Snowflake credits, which are consumed based on the time virtual warehouses are running and the size of the warehouses. The more compute resources are utilized and the longer they run, the more credits are consumed, which directly impacts the cost.

References:

Snowflake Documentation on Virtual Warehouses: Understanding and Managing Warehouse Credit Usage