Google Definition of Courier:
a company or employee of a company that transports commercial packages and documents.
Based on the passage the best answer would be D. An operator is someone is in control, someone who operates on something. And assistant would be someone who helps someone, similar to a secretary. An employee would come in second as the best choice as it is someone hired by someone else but usually a Postman, Mail deliverer can be considered a Courier. The best answer choice is D. A messenger.
