**Core Concept**
Glycogenolysis is the breakdown of glycogen, a complex carbohydrate stored in the liver and muscles, into glucose. This process is essential for providing energy to the muscles, especially during exercise or fasting.

**Why the Correct Answer is Right**
Glycogenolysis is activated in muscles due to the activation of the enzyme glycogen phosphorylase. This enzyme is triggered by the release of epinephrine (adrenaline) from the adrenal glands, which binds to beta-1 adrenergic receptors on the muscle cell surface. The binding of epinephrine to these receptors activates the G-protein coupled receptor pathway, leading to an increase in cAMP levels within the muscle cell. The increased cAMP activates protein kinase A (PKA), which in turn phosphorylates and activates glycogen phosphorylase. This enzyme breaks down glycogen into glucose-1-phosphate, which is then converted to glucose-6-phosphate and eventually released into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glycogenolysis is not primarily activated by insulin, which is a hormone that promotes glycogen synthesis, not breakdown.
**Option B:** This option is incorrect because glucagon is a hormone that promotes glycogenolysis in the liver, not muscles.
**Option C:** This option is incorrect because growth hormone is not directly involved in the activation of glycogenolysis in muscles.

**Clinical Pearl / High-Yield Fact**
Remember that glycogenolysis is an important energy source for muscles during exercise, and its activation is mediated by the sympathetic nervous system through the release of epinephrine.

**Correct Answer:** C. Epinephrine.