Glucagon: The Hormone That Counteracts Insulin's Effects

The correct answer is Glucagon.

Glucagon is a hormone produced by the pancreas that raises blood sugar levels by stimulating the liver to convert glycogen into glucose, which is then released into the bloodstream. Insulin, on the other hand, lowers blood sugar levels by stimulating the liver and muscle cells to take in glucose from the blood. Therefore, glucagon and insulin have opposing actions on blood sugar regulation.