**Core Concept**
Muscle glycogen synthase is a crucial enzyme in the regulation of glycogen synthesis in muscles. The activity of this enzyme is tightly regulated by various factors, including hormonal and nutritional signals. In the context of exercise and recovery, the goal is to replenish muscle glycogen stores by stimulating the activity of glycogen synthase.

**Why the Correct Answer is Right**
The activity of muscle glycogen synthase is primarily regulated by the phosphorylation state of the enzyme. When the muscle is in a resting state, the activity of glycogen synthase is increased by the action of dephosphorylating enzymes, such as protein phosphatase 1 (PP1). PP1 removes phosphate groups from glycogen synthase, thereby activating the enzyme and promoting glycogen synthesis. This process is also influenced by insulin signaling, which is activated in response to carbohydrate intake.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glucagon, a hormone released in response to low blood glucose levels, actually inhibits glycogen synthase activity by promoting its phosphorylation.

**Option B:** This option is incorrect because adrenaline (epinephrine) stimulates glycogenolysis, the breakdown of glycogen to glucose, rather than promoting glycogen synthesis.

**Option C:** This option is incorrect because growth hormone (GH) has a complex effect on glycogen metabolism, but it does not directly increase muscle glycogen synthase activity in resting muscles.

**Clinical Pearl / High-Yield Fact**
Insulin is a key regulator of glycogen synthesis, and its administration or stimulation through carbohydrate intake is essential for promoting muscle glycogen storage after exercise.

**Correct Answer:** C. Protein phosphatase 1 (PP1)