Final answer:
The correct option is d). Carbohydrates are stored as glycogen in the liver and muscles, which acts as an energy reserve and is converted back to glucose when needed.
Step-by-step explanation:
Carbohydrates are stored in the liver and muscles in the form of glycogen. Glycogen is a complex carbohydrate, specifically a highly branched polymer of glucose monomers, functioning as an energy reserve in animals, fungi, and humans. In humans, the liver and muscle cells are the primary sites for glycogen storage. When the body requires glucose and blood glucose levels are low, glycogen is broken down into glucose in a process called glycogenolysis. The liver's stored glycogen helps in maintaining a consistent blood glucose level, while muscle glycogen is used locally within muscle cells during vigorous exercise for energy.