The best data entry control to prevent the incorrect quantity entry would be a limit check, which ensures values are within a certain range.
The data entry control that would best have prevented the cashier from entering a quantity of 20 instead of 2 is a limit check. A limit check involves setting a pre-determined range of acceptable values for a data entry field. In this case, since the inventory indicated 12 items on hand, a limit check could have been set to only accept numbers between 1 and 12 for the quantity field, ensuring that an accidental entry like 20 which exceeds the available stock, would be flagged as an error immediately. This prevents the cashier from proceeding with the incorrect input and prompts them to correct the data before completing the transaction.