The correct answer is option 2: Plants. Carbohydrates are organic compounds that are made up of carbon, hydrogen, and oxygen atoms. They are one of the main sources of energy for living organisms. Plants are the primary source of carbohydrates because they have the ability to perform photosynthesis. During photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose, which is a carbohydrate. Glucose can then be used by the plant for energy or stored as starch for later use. Other organisms, such as animals, obtain carbohydrates by consuming plants or plant products. Option 1, Marine animals, is incorrect because although some marine animals may consume carbohydrates indirectly through eating plants, they are not the primary source themselves. Options 3 and 4, Coal-tar and Crude oil, are also incorrect as they are not sources of carbohydrates.