D. Fossil fuels
1. Nuclear fusion
3. Water cycle
4. Plant life decay
FIGURE IT OUT !
A. Hydropower => Hydro is from greek and means "water". So this would indicate the water cycle.
B. Plants => A plant in general produces its own food. The word only word that has anything to do with producing in the answers is photosynthesis since synthesis is the process of making something. A plant uses light to make sugar, water, and oxygen out of carbon dioxide and water.
C. Sun => You may have to look this up or leave it till last to figure out by elimination, but the sun produces energy by 'fusing' atomic nuclei together hence the term 'nuclear fusion'.
D. Fossil fuels => this is a little misleading since fossil fuels got their name from organisms that lived millions of years ago and have given us fossils today. If instead of being locked in a rock or similar object when it died and the rock forming an impression that lasts millions of years, some animals and plants decay and produce oil, coal, natural gas and other fuels. Also, coal is found where there were previously deep swamps and so in coal there are often fossilized plants and trees known as petrified wood. Petrified wood and coal are the same material, but petrified wood was not exposed to oxygen and was thus 'mummified'.