In which organ is glycogen primarily stored?

Glycogen is primarily stored in the liver because the liver plays a crucial role in regulating blood glucose levels. It stores glycogen and can release glucose into the bloodstream when needed, such as during fasting or physical activity. This makes the liver essential for maintaining energy homeostasis in the body. While muscle tissue also stores glycogen, its main function is to provide energy for muscle contraction rather than for regulating blood sugar levels. The kidneys are involved in filtering blood and maintaining fluid balance, but they do not serve as a significant storage site for glycogen. The heart primarily utilizes fatty acids for energy rather than storing glycogen in large amounts. Thus, the liver is the key organ for glycogen storage, making it the correct answer.