When blood glucose levels increase, a hormone called insulin is released from endocrine cells in the pancreas. The role of insulin is to return blood glucose levels to normal. Which part of this negative feedback loop is the stimulus?

The correct answer is - increasing blood glucose level.

Explanation:

A negative feedback loops takes place in the field of biology specifically in processes of the homeostasis, when the product of reaction leads to decrease in the overall process. Through this the stimulus of a negative feedback loop brings a system to bring the system to equilibrium or stability.

In this provided case increase in glucose level in the blood is acts as the stimulus to release the insulin hormone that return the blood glucose level back to normal.

thus, the correct answer is - increasing blood glucose level.