The correct answer is option 'C', the north-western region.

Explanation:
The north-western region of a country is known for being rich in petroleum and natural gas reserves. Let's explore this in detail.

1. Introduction to the main belt:
- The term "main belt" refers to a geographical area or region that is known for its significant mineral resources.
- In this case, the main belt is associated with petroleum and natural gas, which are important minerals used as sources of energy.

2. Petroleum and natural gas:
- Petroleum, also known as crude oil, is a fossil fuel that is formed from the remains of ancient marine plants and animals.
- Natural gas is a combustible gas composed primarily of methane and other hydrocarbons.
- Both petroleum and natural gas are vital energy sources used for various purposes such as transportation, heating, and electricity generation.

3. Factors determining the location of petroleum and natural gas reserves:
- The formation of petroleum and natural gas deposits is influenced by several geological and environmental factors.
- The presence of sedimentary basins, source rocks, reservoir rocks, and traps are crucial for the accumulation of these minerals.
- These factors determine the location and abundance of petroleum and natural gas reserves.

4. Significance of the north-western region:
- The north-western region of a country is often geologically favorable for the presence of petroleum and natural gas deposits.
- This region may have sedimentary basins, such as the ones found in the Middle East and North America, which are known for their abundant oil and gas reserves.
- Examples of countries with significant petroleum and natural gas reserves in their north-western regions include Saudi Arabia, Iraq, Kuwait, and the United States.

5. Conclusion:
- In conclusion, the main belt where petroleum and natural gas are the main minerals is the north-western region. This region is known for its geological characteristics that favor the accumulation of these valuable energy resources.