Final answer:
Glucagon is released by the pancreas to increase low blood sugar levels through a negative feedback mechanism, inhibiting further glucagon release as levels return to normal.
Step-by-step explanation:
The pancreas releases glucagon when blood glucose levels are low. Glucagon stimulates the liver to convert glycogen back into glucose, increasing blood glucose levels. This is an example of a negative feedback mechanism because the increase in blood glucose levels will inhibit further glucagon release, maintaining homeostatic glucose levels.
In response to the question, the pancreas releases glucagon, which allows (D) low blood sugar levels to be increased by negative feedback.