Final answer:
Glucagon is secreted by β cells to increase blood glucose levels.
Step-by-step explanation:
The correct answer is d) Glucagon secreted by β cells.
The alpha cells in the pancreas secrete glucagon, while the beta cells secrete insulin. Glucagon is responsible for increasing blood glucose levels by promoting the breakdown of stored glycogen in the liver and releasing glucose into the bloodstream. On the other hand, insulin, which is secreted by the beta cells, helps lower blood glucose levels by facilitating the uptake of glucose by cells.