Black soil, also known as regur soil or black cotton soil, is one of the major soil types found in India. It is formed from volcanic activity and is characterized by its dark color, high fertility, and ability to retain moisture. The presence of certain minerals and compounds in black soil contributes to its rich fertility.

The given options (calcium carbonate, magnesium carbonate, potash, and lime) are all substances that are commonly found in black soil. These substances play a crucial role in enhancing the fertility of the soil and supporting the growth of plants.

Here is a detailed explanation of each substance and its significance in black soil:

1. Calcium Carbonate:
- Calcium carbonate is a compound that is abundant in black soil.
- It helps in improving the soil's structure and enhances its ability to retain water.
- Calcium is also important for the growth and development of plant roots.

2. Magnesium Carbonate:
- Magnesium carbonate is another compound found in black soil.
- It helps in maintaining the pH level of the soil, making it suitable for a wide range of crops.
- Magnesium is an essential nutrient for plant growth and is involved in various metabolic processes.

3. Potash:
- Potash refers to various compounds of potassium that are present in black soil.
- Potassium is a vital nutrient for plants as it plays a significant role in their overall growth and development.
- It helps in the synthesis of proteins, activation of enzymes, and regulation of water uptake by plants.

4. Lime:
- Lime, or calcium oxide, is commonly added to black soil as a soil amendment.
- It helps in neutralizing acidic soil conditions, thereby making the soil more suitable for plant growth.
- Lime also provides calcium to the soil, which is important for plant nutrition.

In summary, the presence of calcium carbonate, magnesium carbonate, potash, and lime in black soil contributes to its fertility and ability to support plant growth. These substances help in improving soil structure, maintaining the pH level, and providing essential nutrients to plants. Therefore, the correct answer is option 'C' - black soil.