Final answer:
The most important internal control procedure when duties cannot be segregated is supervision.
Step-by-step explanation:
When duties cannot be segregated, the most important internal control procedure is supervision.
Supervision refers to the oversight and monitoring of employees' activities to ensure that their work is performed accurately and within organizational policies and procedures. It helps to prevent and detect errors, fraud, and other irregularities.
For example, in a retail setting where cashiers handle both cash transactions and inventory management, the supervisor can regularly review sales records, reconcile cash registers at the end of the day, and compare them with inventory records to identify any discrepancies or potential issues.