Question

how is sun considered as ultimate source of fossil fuels

Answer 1

Answer:

The Sun's light energy helps plants to photosynthesise. So in short fossil fuels contain the sun's power stored up inside of dead and decayed organisms that have been pressurized, heated and buried.

Answer 2

Each fossil fuel is a mixture of a great number of carbon-rich energy compounds. Such compounds were developed from compounds found in animal bodies and in plants / trees. These compounds were obtained by animals from the plants / grass which they ate as food.

In the presence of sunlight, plants / grass rendered certain compounds by photosynthesis from carbon dioxide and water.

Thus,

The chemical energy that is stored in fossil fuels comes through plants and animals from the sun. And the primary source of fossil fuel energy is the sun.