Which of the following is a reason to retain part of a payment until the completion of a contract?

Retaining part of a payment until the completion of a contract is primarily to ensure project completion. This practice is commonly known as retention or holdback, and it incentivizes the contractor or service provider to complete the project to the agreed standards and on time. By withholding a portion of the payment, the client retains leverage to ensure that any outstanding work is fulfilled, defects are rectified, or any contractual obligations are met before the final payment is released.

The other options do not directly justify the retention of payment in the context of contract completion. For example, covering labor costs pertains to how payments are processed during the contract's progress rather than an incentive for completion. Tax requirements and equipment depreciation are more related to financial accounting or tax obligations than to the contractual enforcement of completion. The central goal of retaining payment is always to guarantee that all work is satisfactorily finished before the final funds are disbursed.