The major source of glucose released to elevate a low blood glucose level is

Glycogen.

Explanation:

Glycogen may be defined as the polymers of the glucose linked through the glycosidic bonds. The glycogen is the storage form of the glucose present in the animal cells.

The low blood glucose level stimulates the secretion of glucagon hormone. This hormone causes the breakdown of glycogen into the sugar molecules and glucose. Glucose then absorbed by the blood and mainatins the blood glucose level up to the normal level.

Thus, the answer is glycogen.