Identify a conventional source of energy from the following options.

To identify the conventional source of energy from the given options, let us understand the difference between conventional and non-conventional sources of energy.

1. Conventional Sources of Energy: These are traditional sources of energy that have been in use for a long time. They are commonly used for domestic, industrial, and agricultural purposes. Examples include coal, petroleum, natural gas, firewood, and dried cattle dung cakes (traditionally used in rural areas for cooking and heating).

2. Non-Conventional Sources of Energy: These are modern, renewable, and eco-friendly sources of energy that have gained popularity in recent times as alternatives to fossil fuels. Examples include solar energy, wind energy, tidal energy, and modern biomass technologies.

Let us analyze the given options:
- Solar Energy: This is a non-conventional, renewable source of energy harvested using solar panels.
- Wind Energy: This is a non-conventional, renewable source of energy harvested using wind turbines.
- Cattle dung: Dried cattle dung cakes have been traditionally used as a primary source of heat energy for cooking in rural households for centuries. Therefore, it is classified as a conventional source of energy.
- Biomass: While cattle dung is a form of biomass, "Biomass" as a modern category (like biogas or biofuels) is often classified under renewable/non-conventional energy technologies. Among the choices, cattle dung in its traditional direct combustion form is the most classic example of a conventional fuel source used historically.

Thus, the correct option representing a conventional source of energy is Cattle dung.

Therefore, the correct answer is Cattle dung.