Liver glycogen levels rise when blood glucose exceeds normal, signaling storage.

Steps:

• Blood glucose above normal prompts insulin release, increasing liver glycogen synthesis and storage.
• Rising glycogen concentration indicates periods of high blood glucose as excess glucose is converted to glycogen.
• Falling glycogen indicates low blood glucose, with glucagon triggering breakdown to release glucose.
• Identify diagram periods (3 and 5) where glycogen concentration increases, matching high blood glucose.

Why D is correct:

• Periods 3 and 5 show increasing glycogen per the diagram, aligning with insulin-driven storage when blood glucose exceeds normal levels (homeostatic regulation via negative feedback).

Why the others are wrong:

• A: Periods 1 and 6 likely show decreasing glycogen, indicating low blood glucose and breakdown.
• B: Periods 2 and 5 mix decrease (low glucose) with increase (high glucose), inconsistent.
• C: Period 3 shows increase (high glucose), but 4 shows decrease (low glucose), mismatched.

Final answer: D