**Core Concept**
Glucagon is a hormone produced by the pancreas that plays a crucial role in glucose homeostasis. It raises blood glucose levels by stimulating glycogenolysis and gluconeogenesis in the liver.

**Why the Correct Answer is Right**
Glucagon acts on the liver by binding to glucagon receptors, which activates adenylate cyclase and increases intracellular cyclic AMP (cAMP). This leads to the activation of protein kinase A (PKA) and subsequent phosphorylation of key enzymes involved in glycogenolysis and gluconeogenesis. The phosphorylation of glycogen phosphorylase and phosphoenolpyruvate carboxykinase (PEPCK) promotes the breakdown of glycogen and the synthesis of glucose from non-carbohydrate precursors, respectively. This results in an increase in blood glucose levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Insulin is the primary hormone responsible for lowering blood glucose levels, making it an incorrect choice for a condition where glucagon is most effective.
**Option B:** Aldosterone is a mineralocorticoid hormone involved in sodium and water balance, not glucose homeostasis.
**Option C:** Epinephrine is a catecholamine that can raise blood glucose levels, but it does so through different mechanisms, including glycogenolysis and gluconeogenesis in the liver, as well as increased glucose release from adipose tissue.

**Clinical Pearl / High-Yield Fact**
Glucagon is often used in emergency situations, such as severe hypoglycemia or insulin overdose, to rapidly increase blood glucose levels.

**Correct Answer: C. Hypoglycemia.**