Which payment method is allowed for units to purchase supplies?

The correct answer pertains to the purchasing methods authorized for units to acquire supplies. The use of a debit card as the payment method is appropriate because it directly draws from the unit's bank account, ensuring that expenses are closely monitored and within budget. This method provides immediate access to funds while also allowing for better tracking of expenditures compared to cash transactions.

While other options might appear viable in certain contexts, they may not align with the procedural guidelines typically established for purchasing supplies. Credit cards, for instance, might not be as tightly regulated for units, potentially leading to mismanagement or overspending. Cash transactions can often lead to issues with tracking and accountability. A purchasing order is usually a formalized document indicating a commitment to buy goods or services but does not itself represent a direct payment method. Thus, the preference for using a debit card underlines a trend towards maintaining financial discipline and accountability within units when procuring supplies.