Which provides long-term energy storage? glycogen, because it is a polysaccharide glucagon, because it is a complex protein glucose, because it is a monosaccharide cellulose, because it is a complex carbohydrate

The answer is glycogen, because it is a polysaccharide.

Glycogen serves as energy storage in animals, mainly present in liver cells. Unlike the proteins, carbohydrates consist of  more C-H bonds which contain potential energy, so they can serve as energy storage. This is the reason why glycogen provides better energy storage than glucagon. Further, the polysaccharides better provide energy storage because, obviously, they consist of more C-H bonds than the monosaccharides. So, glycogen is better than glucose in energy storage. Although complex carbohydrate, cellulose has rigid structure, giving plant their own structure.