If, and only blood sugar levels rise past a certain point, the cells will release a hormone called insulin. Insulin acts as a/an in a homeostatic feedback loop

When blood glucose level rises beyond the threshold levels, the pancreas secrets hormones insulin. The latter ensures entry of  glucose into the cells through Glut transporters for cellular utilization, therefore reducing the blood levels., and promotes storage as glycogen in the liver and muscles.  

How ever if  the glucose levels drops below the  set point, an hormone glucagon is also produce by the pancreas to cause the liver o withdrawal and breakdown glucose storage as  glycogen back to glucose thus raising the blood glucose level back to the normal levels.

This is an example of negative feedback mechanism, because the increase in the input levels (blood glucose levels) brings about a counter  mechanisms (insulin secretions)as output , to reduce the  elevated   levels by promoting  entry into the cells,, thus bringing the levels to threshold levels.

Thus insulin is acting in a negative feedback mechanism to control blood glucose levels

Explanation:

they use sunlight, water, and carbon dioxide to make sugars - c.