Which one of the following is a non-conventional source of energy?

Conventional sources of energy are those which have been in common use for a long time, such as coal, petroleum, and natural gas.

Non-conventional sources of energy are alternative sources that are generally renewable and have been developed more recently. Examples include solar energy, wind energy, tidal energy, and geothermal energy.

Therefore, tidal energy is a non-conventional source of energy.

Answer: Tidal energy