The alluvial soil found along the banks of the rivers in the Ganga plain is called Khadar. This type of soil is formed due to the deposition of silt and other fine particles by the rivers during flood events. It is one of the most fertile soils in India and is widely used for agricultural purposes.

Here is a detailed explanation of why the correct answer is option 'A' - Khadar:

Formation of Khadar Soil:
- The Ganga plain is characterized by its vast alluvial deposits, which are mainly composed of silt, sand, clay, and other organic matter brought by the rivers.
- When the rivers like Ganga, Yamuna, and their tributaries overflow during the monsoon season, they carry large amounts of sediments and deposit them on the banks.
- These sediments settle down as the floodwaters recede, forming a layer of fertile soil along the riverbanks. This soil is known as Khadar.

Characteristics of Khadar Soil:
- Khadar soil is very fertile due to the presence of fine particles and organic matter in it.
- It has a higher proportion of clay and silt, which gives it good water retention capacity.
- The soil is well-drained and has a high nutrient content, making it suitable for the cultivation of various crops.
- Khadar soil is generally dark in color, indicating its high fertility.

Usage of Khadar Soil:
- Due to its fertility, Khadar soil is extensively used for agriculture in the Ganga plain.
- It is suitable for the cultivation of crops like rice, wheat, sugarcane, vegetables, and pulses.
- The soil's ability to retain water helps in sustaining the crops during dry periods.
- Farmers in the region rely on Khadar soil for their livelihoods as it supports high agricultural productivity.

Comparison with Other Alluvial Soils:
- Bhangar soil is another type of alluvial soil found in the Ganga plain, but it differs from Khadar soil in terms of its formation and characteristics.
- Bhangar soil is older and less fertile compared to Khadar soil. It is formed from older alluvial deposits and is found at a higher elevation.
- Bhur soil, on the other hand, is a sandy soil with low fertility and poor water retention capacity.
- Loess soil is a wind-blown sedimentary soil found in some regions, but it is not found along the banks of rivers in the Ganga plain.

Overall, Khadar soil is the correct answer as it accurately describes the alluvial soil found along the banks of the rivers in the Ganga plain. Its fertility, water retention capacity, and nutrient content make it highly suitable for agriculture in the region.