**Core Concept**
The question tests the understanding of insulin's effect on glucose metabolism, specifically its role in regulating key enzymes involved in gluconeogenesis and glycolysis. Insulin, a **hormone** secreted by the **pancreas**, plays a crucial role in maintaining **blood glucose levels**. It acts on various tissues to facilitate **glucose uptake** and storage.

**Why the Correct Answer is Right**
Insulin decreases the activity of enzymes involved in gluconeogenesis, the process by which glucose is synthesized from non-carbohydrate sources. A key enzyme in this pathway is **glucose-6-phosphatase**, but more broadly, insulin's action results in decreased production of glucose in the liver by inhibiting gluconeogenic enzymes. This action is mediated through **signaling pathways** that ultimately lead to the dephosphorylation and activation or phosphorylation and inhibition of various enzymes.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because insulin actually increases the activity of **glycogen synthase**, promoting glycogen synthesis.
**Option B:** This choice is incorrect because insulin increases the activity of **hexokinase** or **glucokinase** in the liver, facilitating glucose phosphorylation and thus glycolysis.
**Option C:** This choice might refer to an enzyme or process not directly inhibited by insulin's primary actions on glucose metabolism.
**Option D:** Without the specific option text, we can infer that any option suggesting insulin increases activity of enzymes like **phosphoenolpyruvate carboxykinase (PEPCK)** or **fructose-1,6-bisphosphatase** would be incorrect, as insulin inhibits these gluconeogenic enzymes.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that insulin's action on glucose metabolism involves both the promotion of glucose storage (through glycolysis and glycogenesis) and the inhibition of glucose production (through gluconeogenesis). This dual action is crucial for maintaining **normoglycemia**.

**Correct Answer:** D. glucose-6-phosphatase