What is the main source of energy for the earth?
(A) Moon
(B) Plants
(C) Humans
(D) Sun

Hint Energy is very essential for the existence of every being. All planets, stars, and all the other objects in the universe are in constant need of energy. We know different kinds of processes happening in the universe producing energy. But there is only one ultimate source of energy for the planet Earth.

Complete Step by step solution
All things on earth are in constant need of energy in some or other way. Human beings and animals obtain energy from their food. That is indirectly obtained from plants. Plants acquire energy through photosynthesis where they convert light energy from the sun into their useful form of energy. Moon also gets its light from the reflection of sunlight. Hence the only source that constantly gives us energy is the sun.

The answer is: Option (D): Sun

Additional information
In the sun, energy is produced by nuclear fusion. Nuclear fusion occurs when two hydrogen atoms fuse together to form helium atoms. When the hydrogen atoms in the sun get over, the helium atoms will start fusing together to form carbon atoms, then carbon atoms fuse to form silicon and the reaction will end when the final atoms left in the sun become Iron. Since iron is the most stable element, iron will not undergo further fusion. Thus the energy production in the sun will end, resulting in the collapse of the sun and eventually leads to the death of the sun.

Note
The energy from sunlight is converted by plants into their food. That energy is consumed by herbivorous animals. These animals are eaten by carnivorous animals, transferring the energy into them. Again that energy will reach humans through their food.