Final answer:
A carbohydrate is a biological molecule composed mainly of carbon, hydrogen, and oxygen, and has polar characteristics due to the presence of hydroxyl groups.
Step-by-step explanation:
The molecule that is composed primarily of carbon, hydrogen, and oxygen and is polar is a carbohydrate. Carbohydrates are key biological macromolecules that serve various functions in the body such as energy storage and providing structure. They consist of carbon (C), hydrogen (H), and oxygen (O) atoms, usually with a hydrogen:oxygen atom ratio of 2:1 as in water, and often have polar properties due to the presence of numerous hydroxyl (OH) groups which make them hydrophilic. Some common examples of carbohydrates include sugars, starches, and fibers. In the context of cellular structures, carbohydrates are found on the outside surface of plasma membranes where they are attached to proteins and lipids.