Final answer:
Requiring a Purchase Requisition mitigates the threat of unauthorized purchases, serving as a control mechanism to ensure necessary and authorized expenditure in a business.
Step-by-step explanation:
When companies require a Purchase Requisition be prepared and approved within the Expenditure transaction cycle, the main threat this process mitigates is c) Unauthorized purchases. The requisition document serves as a control mechanism to ensure that purchases are made only when necessary and that they are properly authorized before any expenditure is incurred. By having a formal process in place, businesses can prevent employees from buying goods or services that are not approved or necessary for business operations, which can lead to overspending or fraudulent transactions.