During heavy exercise, the breathing rate can increase upto ........... per minute.

The correct option is D 25 times
During exercise, your lungs and respiratory system must provide more oxygen to the blood. You will breathe harder and faster because: Respiratory muscles are stimulated by sympathetic nerves in order to increase the rate of breathing. If the exercise is intense, breathing rates may increase from a typical resting rate of 15 breaths per minute up to 40 – 50 breaths per minute. Hence, during heavy exercise, the breathing rate can increase up to 25 times.

So, the correct option is '25 times'.