The correct answer is option 'B', the Mississippi River. The Mississippi River is an important river in the United States that complements the natural rainfall in the Prairies and provides irrigation facilities. Let's explore why this is the correct answer in more detail.

The Prairies and their Climate:
The Prairies are a region in North America known for their flat grasslands and fertile soils. They are located in the central part of the continent, primarily in the United States and Canada. The climate in the Prairies is characterized by hot summers and cold winters, with moderate precipitation.

The Importance of Rainfall in Agriculture:
Rainfall is crucial for agriculture as it provides water for crops to grow. However, the natural rainfall in the Prairies may not always be sufficient for the agricultural needs of the region. This is especially true during dry periods or droughts when the amount of rainfall is significantly reduced.

The Role of the Mississippi River:
The Mississippi River, one of the longest rivers in North America, plays a vital role in complementing the natural rainfall in the Prairies. Here's how it provides irrigation facilities:

- Water Source: The Mississippi River acts as a major water source for the Prairies. It supplies water to the region, especially during periods of low rainfall.

- Irrigation: The river provides irrigation facilities to farmers in the Prairies. Irrigation involves supplying water to crops through artificial means, such as canals, ditches, or sprinklers. This helps to ensure that crops receive sufficient water for growth and development, even when natural rainfall is limited.

- Transportation: The Mississippi River also serves as a transportation route, allowing farmers to transport their agricultural products to markets and consumers. This transportation network contributes to the economic development of the region.

In conclusion, the Mississippi River complements the natural rainfall in the Prairies and provides irrigation facilities. Its water supply and irrigation capabilities are essential for agriculture in the region, helping to ensure the growth and productivity of crops even during periods of low rainfall.