Okay, let's tackle this question. The question is about which enzyme is activated by a decrease in the insulin to glucagon ratio. Hmm, so first, I need to recall what insulin and glucagon do. Insulin is an anabolic hormone, promoting glucose uptake and storage, while glucagon is catabolic, promoting glucose release into the blood.

When insulin levels decrease and glucagon increases, the ratio of insulin to glucagon drops. This would signal the body to mobilize energy stores. So enzymes involved in breaking down glycogen or fatty acids might be activated. Let's think about the key enzymes here.

In the liver, glycogenolysis is mediated by glycogen phosphorylase. Insulin inhibits this enzyme, while glucagon activates it. So a decrease in the insulin/glucagon ratio would mean more glucagon activity, leading to activation of glycogen phosphorylase. Another possibility is hormone-sensitive lipase in adipose tissue, which is activated by glucagon to break down triglycerides. But the question is about an enzyme activated by the ratio change.

Wait, the question says "enzyme activated by decrease in Insulin: glucagon ratio". So when insulin is low and glucagon is high, which enzyme is turned on? Glycogen phosphorylase in the liver would be activated. Also, hormone-sensitive lipase in fat cells. But which one is more commonly associated with this ratio? The question might be pointing towards the liver enzyme.

Looking at the options (though they aren't provided here), the correct answer is likely glycogen phosphorylase. The options might include other enzymes like hexokinase, phosphofructokinase, or maybe acetyl-CoA carboxylase. Hexokinase is involved in glycolysis and is inhibited by glucose-6-phosphate, not directly by insulin/glucagon ratio. Acetyl-CoA carboxylase is regulated by insulin and involved in fatty acid synthesis, but it would be inhibited when glucagon is high.

So the key here is to remember that a low insulin/glucagon ratio activates enzymes that break down energy stores. Glycogen phosphorylase is the main one in the liver, leading to glycogenolysis. Therefore, the correct answer should be glycogen phosphorylase. The other options would be enzymes that are inhibited when glucagon is high, like acetyl-CoA carboxylase. Let me make sure.

Yes, glycogen phosphorylase is activated by glucagon through the cAMP pathway. So when insulin is low and glucagon is high, the enzyme is activated. That makes sense. So the correct answer is glycogen phosphorylase. The other enzymes like hexokinase or phosphofructokinase are more about glycolysis and not directly regulated by these hormones in the same way. Hormone-sensitive lipase is another possibility, but the question might be focusing on liver enzymes. Since the options aren't given, but the correct answer is likely glycogen phosphorylase.

**Core Concept**
The insulin-to-glucagon ratio regulates metabolic pathways. A decrease in this ratio (low insulin, high glucagon) activates catabolic processes like glycogenolysis and gluconeogenesis in the liver.

**Why the Correct Answer is Right**
Glycogen phosphorylase is the key enzyme activated by a low insulin/glucagon ratio

✓ Correct Answer: D. Glucose 6 phosphatase