Hello!
The answer is the volume will decrease.
Why?
It's well known that while the pressure and temperature remain constant, the volume of the gas will keep constant, but we have a particular case here since there is a respirometer and assuming that it's being used by a living organism, the gas will decrease even if the pressure and the temperature remains constant.
So, why the volume of gas will decrease? We must remember that the living organisms respiration consists in taking in oxygen and release CO2, which it's not allowed to remain inside in the air that is contained in a respirometer.
A respirometer is a device used to measure the rate of oxygen consumption by a living organism. It works measuring the exchange of oxygen and carbon dioxide.
Have a nice day!