The correct option for the type of letters that are delivered through the normal mail or by airmail express service is D. airmail letters. Explanation of the Correct Answer Airmail Letters (Option D): - Airmail letters refer specifically to letters that are sent via air transport, which is typically faster than ground transport. This service is used for both domestic and international mail. - Airmail is a common choice for sending letters that need to reach their destination quickly, especially over long distances. - The term "airmail" encompasses letters that are sent through regular postal services that utilize air transport, which aligns with the question's description of letters delivered through normal mail or express service. Why the Other Options are Incorrect A. Express Letters: - Express letters are typically associated with expedited delivery services that guarantee faster delivery times, often within a specific timeframe (like overnight or same-day delivery). - While express services may use air transport, they are not synonymous with airmail. Express services often come with additional costs and tracking features, which are not necessarily part of standard airmail services. B. Registered Letters: - Registered letters are a type of mail that provides additional security and tracking. They are often used for important documents or valuable items. - While registered letters can be sent via airmail, they are not defined by the mode of transport (air vs. ground) but rather by the added security and tracking features. Therefore, they do not fit the description of letters delivered through normal mail or express service. C. Inland Letters: - Inland letters refer to letters sent within the same country. This term does not specify the mode of transport (air or ground) and is more about the geographical aspect of the delivery. - While inland letters can be sent via normal mail, they do not encompass the express service aspect mentioned in the question. Thus, they are not the correct answer. Summary of Key Points

• Airmail letters are specifically designed for fast delivery via air transport, making them the correct answer.
• Express letters focus on expedited delivery but are not limited to air transport.
• Registered letters provide security and tracking but do not define the transport method.
• Inland letters refer to domestic mail without specifying the delivery speed or method.

Revision Summary

• Airmail letters are sent via air transport and can be normal or express.
• Express letters guarantee faster delivery but are not limited to air transport.
• Registered letters offer security and tracking, not defined by transport mode.
• Inland letters are domestic and do not specify delivery speed or method.