Which of the following organic compounds is the main source of energy for living things?

• A. Nucleic acids
• B. Lipids
• C. Proteins
• D. $\text{Carbohydrates}$

Answer: (D)

Verified step by step guidance

1. Understand the role of each class of organic compounds in living organisms: nucleic acids store genetic information, lipids serve as long-term energy storage and structural components, proteins perform various functions including catalysis and structure, and carbohydrates primarily provide energy.
2. Recall that carbohydrates are composed of carbon, hydrogen, and oxygen, typically in a ratio of 1:2:1, and include sugars like glucose, which is a key energy source.
3. Recognize that glucose, a simple carbohydrate, is the primary molecule used by cells to produce ATP through cellular respiration, making carbohydrates the main immediate source of energy.
4. Note that while lipids store more energy per gram, carbohydrates are more readily mobilized and used quickly by cells for energy.
5. Conclude that among the options given, carbohydrates are the main source of energy for living things due to their role in providing quick and accessible energy.