Final answer:
The contracting officer can only terminate a purchase order for convenience before the delivery of goods or services.
Step-by-step explanation:
The correct answer is a) Before the delivery of goods or services.
Terminating a purchase order for convenience means ending the contract without any fault on the contractor's part. It is done by the contracting officer when there is a need to stop the procurement process or change the specifications. This can be done before the delivery of goods or services as it allows the contracting officer to avoid unnecessary costs and obligations.