Concept:NIPOST provides a "poste restante" type of service for people who do not have a fixed address or who are travelling, so their mail is kept at a named post office until they call for it.
Explanation:In this service, the sender addresses the letter or parcel to a particular post office instead of to a home or office address.
The post office receives and safely keeps the mail, waiting for the addressee to come and collect it.
The addressee must present valid identification at that post office before the letter is handed over.
This exactly matches the situation in the question: letters are addressed to any post office to await collection by the addressee.
This arrangement is therefore known as the free post service in NIPOST.
Answer:C. free post service