The type of letters that are delivered through the normal mail or by airmail express service is referred to as
- express letters
- registered letters
-
inland letters
- airmail letters
The correct answer is: C
Explanation
The type of letters that are delivered through the normal mail or by airmail express service is referred to as inland letters. Inland letters are letters that are sent within the same country. They can be delivered through the normal mail service or by airmail express service, depending on the sender's preference and the urgency of the letter.