Final answer:
Glycogen phosphorylase is inhibited by both ATP and glucose 6-phosphate, as their presence indicates that the cell has sufficient energy and does not need to mobilize glucose from glycogen.
Step-by-step explanation:
The statement that is true of glycogen phosphorylase is: d) It is inhibited by glucose 6-phosphate and by ATP. Glycogen phosphorylase catalyzes the first step in glycogenolysis, which is the hydrolysis of glycogen to glucose-1-phosphate. In a well-fed state, when energy levels are high, the presence of ATP and glucose 6-phosphate signals that the cell does not require additional glucose from glycogen breakdown. Therefore, both ATP and glucose 6-phosphate act as allosteric inhibitors of glycogen phosphorylase, preventing unnecessary release of glucose from glycogen stores.