The correct answer is option 4, solar energy. To understand why solar energy is considered a non-conventional source of energy, let`s look at the other options first.

Option 1, petroleum: Petroleum is a conventional source of energy as it has been widely used for many years. It is a fossil fuel that is formed from the remains of ancient plants and animals.

Option 2, coal: Similar to petroleum, coal is also a conventional source of energy. It is also a fossil fuel that is formed from the remains of ancient plants in swampy environments.

Option 3, radioactive elements: Radioactive elements, such as uranium and plutonium, are not considered a conventional source of energy either. They are used in nuclear power plants to generate electricity through nuclear fission.

Now, let`s come back to option 4, solar energy: Solar energy is a non-conventional source of energy because it is derived from the sun. It does not involve the use of fossil fuels or nuclear energy. Solar energy is harnessed using solar panels, which convert sunlight into electricity. It is considered a renewable and sustainable source of energy as the sun`s energy is virtually infinite and does not produce harmful pollutants.

Therefore, the correct answer is option 4, solar