Question

What hormone stimulates the liver to release glucose into the blood?

Answer 1

glucagon

Explanation:

Glucagon and insulin are complementary hormones - homeostasis requires that there be negative feedback inhibition whereby the desired result of a hormone deactivates the production of said hormone.
Under normal conditions when your blood sugar rises above the baseline concentration in your bloodstream, insulin production is stimulated which lowers the blood sugar - as the blood sugar lowers so does insulin production. A diabetic must inject an external source of insulin to maintain homeostasis.

When blood sugar drops below baseline, the body will release glucagon which stimulates the decomposition of glycogen in the liver and the release of stored glucose into the bloodstream. Diabetics will eat something sugary to increase their blood sugar rather than inject a second hormone in glucagon.
(accidental injection of the wrong hormone could be lethal)