In this general knowledge odd one out question on fuels and chemicals, choose the term that is not a common fuel: Mustard gas, Gasoline, Diesel and Natural gas.

Introduction / Context:
This odd one out question is based on basic general knowledge about fuels and chemicals. Three of the given options are widely used as fuels for vehicles, heating or cooking, while one is a dangerous chemical warfare agent. Distinguishing between everyday energy sources and harmful chemical agents is the key to identifying the odd one out.

Given Data / Assumptions:

• The options are Mustard gas, Gasoline, Diesel and Natural gas.
• Gasoline is used as a fuel in many vehicles.
• Diesel is also a fuel for engines and transport.
• Natural gas is used as a fuel for cooking, heating and power generation.
• Mustard gas is known as a toxic chemical used in warfare, not as a commercial fuel.

Concept / Approach:
We classify the given items into two categories: fuels and non fuel chemical agents. If three items are commercially used fuels and one is not, the non fuel item becomes the odd man out. The question relies on general awareness, not on any numerical pattern or scientific formula.

Step-by-Step Solution:
Step 1: Gasoline (also called petrol) is a refined petroleum product used as a common fuel for cars and motorcycles. Step 2: Diesel is another refined fuel derived from crude oil, widely used in trucks, buses, generators and many heavy vehicles. Step 3: Natural gas is mainly composed of methane and is used as a fuel for cooking, heating and sometimes electricity generation. It is also compressed as CNG for vehicles. Step 4: Mustard gas, however, is not a fuel. It is a toxic chemical warfare agent that causes severe blistering and injury. It is not used for energy or transport, but is known as a harmful weapon. Step 5: Therefore three options are fuels and one is a chemical warfare agent. Mustard gas is the odd one out.

Verification / Alternative check:
To verify, imagine each item in a practical context. Gasoline, diesel and natural gas are all sold at fuel stations or supplied for heating and cooking. People routinely buy them to run vehicles or appliances. Mustard gas is never sold or used as a fuel in normal life; instead, it is associated with military history and chemical warfare conventions. This real world perspective confirms the classification difference.

Why Other Options Are Wrong:
Gasoline is wrong as the odd one out because it is a standard fuel used in countless vehicles. Diesel is wrong as the odd one out because it is another widely used fuel for engines and generators. Natural gas is wrong as the odd one out because it is again a common fuel for domestic and industrial purposes.

Common Pitfalls:
Some candidates may be misled by the word gas in both Mustard gas and Natural gas and focus only on the physical state rather than the purpose. However, the key difference is between a fuel and a toxic agent. Another trap is to think that all items are gases, but diesel and gasoline are liquids, so this does not give a clear majority pattern. The safest classification in this question is fuel versus weapon.

Final Answer:
The only option that is a chemical warfare agent rather than a fuel, and therefore the odd one out, is Mustard gas.