What is the function on an ovary in a flower?

The ovary is the expanded inferior part of the carpel. It contains the ovules waiting to be fertilized by pollen. 
The function of an ovary is to hold egg cells in a flower to use when beginning the reproductive cycle.

C. a rise in blood carbon dioxide concentration

Explanation:

Negative feed back mechanism refers to the control mechanism in which a product of a particular pathway controls the switch on and switch off of that particular pathway.

Since breathing increases the blood oxygen concentration and lowers carbon dioxide concentration and pH. The sensors that regulate the breathing should respond to oxygen concentration as the carbon dioxide is the by-product of respiration and is responsible for increasing blood pH. Thus  increased Carbon dioxide levels in blood is the only factor which will trigger breathing.