**Core Concept**
Phosphorylase b is a key enzyme in glycogenolysis, the breakdown of glycogen to glucose-1-phosphate. In muscle, phosphorylase b is kept in an inactivated state by a regulatory mechanism that prevents excessive glycogen breakdown. This mechanism is crucial for maintaining energy homeostasis in muscle cells.

**Why the Correct Answer is Right**
The inactivated state of phosphorylase b is maintained by its association with glycogen. In muscle, glycogen is tightly bound to phosphorylase b, preventing its activation by phosphorylation. This association is mediated by the glycogen-binding site on the phosphorylase b molecule. When glycogen is broken down, the enzyme is released from glycogen and can be activated by phosphorylation. This regulatory mechanism ensures that glycogen breakdown is tightly coupled to the energy needs of the muscle cell.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because phosphorylase b is not regulated by insulin in muscle. Insulin actually promotes glycogen synthesis, not breakdown.

**Option B:** This option is incorrect because phosphorylase kinase is the enzyme responsible for phosphorylating and activating phosphorylase b, not keeping it in an inactivated state.

**Option C:** This option is incorrect because glucose-6-phosphatase is an enzyme involved in gluconeogenesis and glycogenolysis, but it is not responsible for regulating phosphorylase b in muscle.

**Clinical Pearl / High-Yield Fact**
In muscle, the regulation of phosphorylase b is a classic example of allosteric regulation, where the binding of glycogen to the enzyme prevents its activation by phosphorylation. This mechanism is essential for maintaining energy homeostasis in muscle cells and preventing excessive glycogen breakdown.

**Correct Answer:** D. Glycogen