The correct answer is option 'A' - Yalu River.

Explanation:
The Yalu River forms the border between North Korea and China. It is a major river in East Asia and serves as a natural boundary between the two countries. Here are some key details about the Yalu River:

1. Location: The Yalu River flows along the border between North Korea and China. It originates from the Changbai Mountains in China's Jilin Province and forms part of the border between Liaoning Province in China and North Korea.

2. Length: The Yalu River has a total length of approximately 795 kilometers (494 miles). It flows from its source in China, through North Korea, and eventually empties into the Yellow Sea.

3. Importance: The Yalu River is of great strategic importance for both North Korea and China. It not only serves as a natural boundary but also plays a crucial role in the economic and cultural exchanges between the two countries.

4. Border Crossing: The Yalu River serves as a major crossing point between North Korea and China. There are several bridges that span the river, allowing people and goods to pass between the two countries. The most well-known bridge is the Sino-Korean Friendship Bridge, which connects the Chinese city of Dandong with the North Korean city of Sinuiju.

5. Natural Resources: The Yalu River basin is rich in natural resources, including coal, iron ore, and other minerals. These resources have contributed to the economic development of the region and have been a source of trade between North Korea and China.

In conclusion, the Yalu River forms the border between North Korea and China. It is a significant river in East Asia, serving as a natural boundary and playing a crucial role in the economic and cultural exchanges between the two countries.