Separate authorization for input of transactions. This control would have best prevented this type of fraud in a retail environment by ensuring that the warehouse employee who handles the inventory items does not have the authority to enter adjustments to the inventory system. This would create a segregation of duties that would reduce the risk of collusion and concealment of theft.

The other options are not as effective as option A in preventing this type of fraud. Option B, statistical sampling of adjustment transactions, is a detective control that may help identify fraudulent transactions after they have occurred, but it does not prevent them from happening in the first place. Option C, unscheduled audits of lost stock lines, is also a detective control that may reveal discrepancies between the physical and recorded inventory, but it does not address the root cause of the fraud. Option D, an edit check for the validity of the inventory transaction, is a preventive control that may help verify the accuracy and completeness of the transaction data, but it does not prevent unauthorized or fraudulent adjustments.