Okay, let's tackle this question. The question is about an enzyme that gets activated when there's a decrease in the insulin to glucagon ratio. Hmm, insulin and glucagon are the two main hormones regulating glucose metabolism. Insulin lowers blood glucose by promoting storage, while glucagon raises it by stimulating breakdown.

So, if the ratio of insulin to glucagon decreases, that means insulin is lower relative to glucagon. That would happen in a fasting state or during stress, where glucagon levels rise. The body needs to increase glucose availability. The key enzymes involved in glycogen breakdown are glycogen phosphorylase and glucose-6-phosphatase. But wait, in the liver, glycogenolysis is mainly driven by glucagon. Glycogen phosphorylase is the enzyme activated by glucagon, which breaks down glycogen into glucose-1-phosphate.

Wait, but what about gluconeogenesis? Glucagon also promotes gluconeogenesis. The enzyme phosphoenolpyruvate carboxykinase (PEPCK) is a key enzyme in gluconeogenesis. So, when glucagon is high, PEPCK is upregulated. But is PEPCK directly activated by the insulin/glucagon ratio? Or is it more about transcriptional regulation?

Also, the hormone-sensitive lipase is activated by glucagon and epinephrine to break down triglycerides, releasing fatty acids. But that's more about lipolysis than glucose production.

So, the enzyme activated by the decrease in insulin/glucagon ratio (i.e., increased glucagon) would be glycogen phosphorylase in the liver, which is activated via the cAMP pathway triggered by glucagon. Alternatively, maybe glucose-6-phosphatase is involved in the final step of glycogenolysis and gluconeogenesis. But glucose-6-phosphatase is constitutively present, not enzyme that's activated by hormones.

Another possibility is the enzyme pyruvate carboxylase, which is a key enzyme in gluconeogenesis. However, its activity is more related to the availability of acetyl-CoA from fatty acid oxidation, which is promoted by glucagon. But is it directly activated by the hormone? Or is it regulated at the transcriptional level?

Wait, the question is about an enzyme activated by the decrease in insulin/glucagon ratio. So, the correct answer is likely glycogen phosphorylase. When glucagon levels rise (due to low insulin), it activates adenylate cyclase, increasing cAMP, which activates protein kinase A (PKA). PKA then phosphorylates and activates glycogen phosphorylase, leading to glycogen breakdown.

**Core Concept**
The question examines hormonal regulation of glucose metabolism, specifically the effect of **insulin** (anabolic, promotes storage) and **glucagon** (catabolic, promotes breakdown) on enzyme activity. A **decrease in insulin/glucagon ratio** (e.g., fasting, stress) activates catabolic pathways to increase blood glucose.

**Why the Correct Answer is Right**
The enzyme **glycogen phosphorylase** is activated by **glucagon** via a **cAMP-dependent pathway**. When insulin levels fall and glucagon rises, **adenylate cyclase** is activated, increasing **cAMP** and **protein kinase A (PKA)**

✓ Correct Answer: D. glucose 6 phosphatase