The correct answer is C. $P_1V_1 = P_2V_2$. This is the ideal gas law, which states that
for a fixed amount of an ideal gas kept at a fixed temperature, pressure and volume are inversely proportional.Option A is incorrect because it does not take into account the volume of the gas. Option B is incorrect because it does not take into account the pressure of the gas. Option D is incorrect because it does not take into account the temperature of the gas.
The ideal gas law is a good approximation for many gases at low pressures and high temperatures. It is not a good approximation for gases at high pressures or low temperatures, where the gas molecules interact with each other in a more complex way.