* A stock rule is a record that defines the minimum and maximum quantity of a model or an asset in a stockroom12.
* The Stock Rule Runner is a scheduled job that runs every hour and checks the stock levels of models and assets in stockrooms against the stock rules13.
* If the stock level of a model or an asset is less than the minimum quantity specified in the stock rule, the Stock Rule Runner creates a stock order to replenish the stock from another stockroom or from a vendor13.
* Therefore, the correct answer is A. When stock is less than the threshold specified in the stock rule.
References: 1: Use a hardware asset request flow - ServiceNow 2: Hardware Asset Inventory Management - ServiceNow 3: Stock Rules in ServiceNow - GlideFast ServiceNow