What helped the Khans maintain communication over long distances during the height of the Mongol Empire? A.
The Khans built an extensive network of roads throughout the territories that they conquered.

B.
The Khans required their subjects to learn to read and write so they could understand the empire's laws.

C.
The Khans developed a postal system that allowed them to efficiently send messages throughout the empire.

D.
The Khans forced people to learn Mongolian so that they could speak to people from other regions.

C.  The Khans developed a postal system that allowed them to efficiently send messages throughout the empire.

Explanation:

Communication over long distances during the height of the Mongol Empire was done through postal-relay horse stations called Örtöö. These postal relay horse station spreads throughout the empire which is the main source of communication between khans and their generals and army officers. This source of communication is the best at that time which is considered as the fastest mean of communication.

the answer for 3 is b and the answer for 4 is d

answer; taking up arms to defend the nation;