Concept:Letters are grouped according to their delivery method and the area they cover.
Explanation:Inland letters are posted and delivered within the same country.
They can move through the ordinary postal route, known as normal mail.
They can also be sent by airmail express service when quicker delivery is required.
Therefore, inland letters are not limited to only one special kind of dispatch.
Express letters are treated as urgent and given special handling attention.
Registered letters are recorded for safety and compensation if they get lost.
Airmail letters are those transported by air, mostly to foreign destinations.
Since the letter may pass through the regular mail or an airmail express service inside one nation, it is best described as an inland letter.
Answer:C. Inland letters.