A depleting source of energy is fossil fuels.
Fossil fuels are buried flammable geologic deposits of organic substances such as dead plants and animals that got deposited under several thousand feet of silt. These deposits decayed with the passage of time and got converted to natural gas, coal and petroleum due to the extreme heat and pressure inside the earth’s crust. They are also known as non-renewable sources of energy as it takes a very long time for it to replenish.
Therefore, the correct option is (D)