Which reason best explains why living things need carbon? carbon is a source of energy. carbon reduces global temperatures. carbon is an important solvent. carbon is everywhere.

The answer is Carbon is a source of energy. 

Plants absorb carbon dioxide and make carbohydrates in a process of photosynthesis. Carbohydrates have many C-H bonds which breaking down releases such amount of energy. Also, lipids are the macromolecules with the highest number of carbon-hydrogen (C-H) bonds, so they contain more C-H bonds than carbohydrates. So, carbon is the part of macromolecules containing potential energy in the form of C-H bonds. At the end, carbon is the source of energy.