When the volume and number of particles of a gas are constant which of the following is also constant

The product of the pressure and volume.

In Boyel's experiment he changed the pressure and noticed the change in volume with respect to pressure keeping temperature and moles of gas constant

So here we have three kinds of quantities

a) controlled variables : Temperature and moles as they have to be kept constant

b) independent variable : pressure as we are varying it and the volume is changing with respect to pressure

c) Dependent variable: Volume , as it depends upon the change in pressure.