**Core Concept**
Insulin is a hormone secreted by the pancreas that plays a crucial role in glucose regulation. By binding to its receptor, insulin triggers a cascade of intracellular signaling pathways that ultimately lead to the uptake of glucose by cells, glycogen synthesis, and inhibition of gluconeogenesis. One of the key enzymes involved in gluconeogenesis is pyruvate kinase, but insulin actually inhibits gluconeogenic enzymes like glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. However, a more direct effect of insulin is the inhibition of the enzyme fructose-1,6-bisphosphatase.

**Why the Correct Answer is Right**
Fructose-1,6-bisphosphatase is a key enzyme in gluconeogenesis, responsible for the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate. Insulin inhibits this enzyme through the activation of protein phosphatase 2A, which dephosphorylates and inactivates fructose-1,6-bisphosphatase. This inhibition of fructose-1,6-bisphosphatase reduces gluconeogenesis and contributes to the decrease in blood glucose levels seen after insulin administration.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while insulin does have effects on other enzymes involved in glucose metabolism, such as pyruvate kinase, it does not directly decrease the activity of this enzyme.

**Option B:** This option is incorrect because insulin actually increases the activity of phosphofructokinase-1, a key enzyme in glycolysis, rather than decreasing it.

**Option C:** This option is incorrect because insulin does not have a direct effect on the activity of glucose-6-phosphatase, although it does inhibit gluconeogenesis.

**Clinical Pearl / High-Yield Fact**
Insulin's effects on glucose metabolism are complex and multifaceted, but one key point to remember is that insulin inhibits gluconeogenesis by activating protein phosphatase 2A, which dephosphorylates and inactivates key enzymes like fructose-1,6-bisphosphatase.

**Correct Answer:** D.