**Core Concept**
Adrenaline, a catecholamine hormone, plays a crucial role in regulating the body's 'fight or flight' response. During this response, adrenaline stimulates the breakdown of glycogen to glucose in the liver and skeletal muscles, a process known as glycogenolysis. This process is mediated by a specific enzyme that is activated by adrenaline.

**Why the Correct Answer is Right**
Adrenaline acts on the enzyme **glycogen phosphorylase** in glycogenolysis. Glycogen phosphorylase is a key enzyme responsible for breaking down glycogen into glucose-1-phosphate. Adrenaline binds to **beta-adrenergic receptors** on the surface of hepatocytes and muscle cells, triggering a signaling cascade that ultimately activates glycogen phosphorylase. This activation leads to the breakdown of glycogen and the release of glucose into the bloodstream.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as adrenaline does not act on **phosphofructokinase**, an enzyme involved in glycolysis, not glycogenolysis.
* **Option B:** This option is incorrect as adrenaline does not act on **glucose-6-phosphatase**, an enzyme involved in gluconeogenesis, not glycogenolysis.
* **Option C:** This option is incorrect as adrenaline does not act on **hexokinase**, an enzyme involved in glycolysis, not glycogenolysis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that adrenaline stimulates glycogenolysis in the liver and skeletal muscles, but not in the brain, where glucose is always the primary fuel source.

**Correct Answer:** C. Glycogen phosphorylase