Introduction:
In the question, we are asked to identify the fuel that is not used for cooking. Let's analyze each option to determine the correct answer.

Analysis:
a) Coal: Coal is a fossil fuel that has been used for cooking and heating for centuries. In certain regions, coal is still used as a cooking fuel, although its usage has significantly decreased due to environmental concerns.

b) Petrol:
Petrol, also known as gasoline, is primarily used as a fuel for vehicles and machinery. It is highly flammable and not suitable for cooking purposes. Petrol is a liquid fuel that is obtained from crude oil through a refining process.

c) Wood:
Wood is a common fuel used for cooking in many households around the world. It is easily available and provides a steady source of heat when burned. Wood is often used in stoves, fireplaces, and traditional cooking methods.

d) Cow dung:
Cow dung, also known as cow manure, is a biomass fuel that is widely used in rural areas for cooking and heating. It is a byproduct of animal husbandry and agriculture. Cow dung is dried and formed into cakes or patties, which are then used as fuel in traditional stoves.

Conclusion:
Based on the analysis above, the correct answer is option 'B' - Petrol. Petrol is not commonly used for cooking as it is primarily used as a fuel for vehicles and machinery.