Natural fuels are fuels that are obtained from nature. They are derived from natural resources and are considered to be renewable sources of energy. These fuels are used for various purposes, including heating, cooking, transportation, and electricity generation.

Natural fuels can be further classified into two categories: primary fuels and secondary fuels.

Primary fuels:
Primary fuels are the fuels that are directly obtained from nature without any processing or conversion. They are the primary sources of energy and can be used directly for various purposes. Some examples of primary fuels include:

1. Wood: Wood is one of the oldest and most commonly used natural fuels. It is obtained from trees and is used for heating and cooking purposes.

2. Coal: Coal is a fossil fuel that is formed from the remains of plants that lived millions of years ago. It is a widely used fuel for electricity generation and industrial processes.

3. Petroleum: Petroleum, also known as crude oil, is a liquid fossil fuel that is extracted from underground reservoirs. It is refined to produce various fuels such as gasoline, diesel, and jet fuel.

4. Natural gas: Natural gas is a gaseous fossil fuel that is primarily composed of methane. It is found in underground reservoirs and is used for heating, cooking, and electricity generation.

Secondary fuels:
Secondary fuels are the fuels that are derived from primary fuels through various processes. They are produced by converting or processing primary fuels. Some examples of secondary fuels include:

1. Gasoline: Gasoline is a fuel that is produced by refining petroleum. It is widely used as a fuel for automobiles.

2. Diesel: Diesel is another fuel that is derived from petroleum. It is used in diesel engines, which are commonly found in trucks, buses, and some cars.

3. Liquefied petroleum gas (LPG): LPG is a mixture of propane and butane gases. It is derived from natural gas or petroleum and is used for heating, cooking, and as a fuel for vehicles.

In conclusion, natural fuels are obtained from nature and can be classified into primary fuels and secondary fuels. They are renewable sources of energy and play a crucial role in meeting our energy needs.

Fuels obtained from nature are called natural fuels.
Natural fuels are substances that are formed from organic matter over millions of years through natural processes. They are derived from renewable or non-renewable sources found in nature. Examples of natural fuels include fossil fuels such as coal, oil, and natural gas, as well as biomass, which includes wood, agricultural waste, and animal waste.
Here is a detailed explanation of the options given:
A. Primary fuels: This term refers to the main energy sources that can be directly used, including both natural and synthetic fuels. It is a broader term that encompasses both natural and human-made fuels.
B. Natural fuels: This is the correct answer. Natural fuels are obtained from nature and include fossil fuels and biomass.
C. Exhaustible fuels: This term refers to fuels that are finite in quantity and can be depleted over time. Fossil fuels are considered exhaustible as they take millions of years to form and are being consumed much faster than they are being replenished.
D. Inexhaustible fuels: This term refers to fuels that are considered unlimited in quantity and can be replenished continuously. Examples include solar energy, wind energy, and hydroelectric power.
In conclusion, the correct answer is B. Natural fuels are obtained from nature and include fossil fuels and biomass.