Answer:
pressure
Why is pressure constant in Charles’s Law?
We build a "ideal experiment" with only one independent variable changing. So Charles Law is intended to investigate the influence of V (dependent variable) and temperature T (independent variable), while holding everything else constant, particularly pressure P. We may alternatively consider temperature T as a dependent variable and V as an independent variable (like when you compress a gas, and see what is the change of temperature, although it is very difficult to practically compress a gas while keeping the pressure constant).
If everything is kept at 1 atm, it is simple to keep pressure constant in the lab. That is, in the absence of any hard-wall containers, we may reasonably infer that the pressure is constant.
Other rules were created to investigate processes with V constant (isometric), T constant (isothermal), and adiabatic (Q = 0). Of course, the majority of them eventually unite to form a single ideal gas law.
Thanks,
Eddie