Which atmospheric gas has the greatest effect on the weather conditions associated with an air mass?

Among the gases mentioned if the list of options, nitrogen, oxygen, methane and water vapor, water vapor has the greatest effect on the weather conditions. The reason is that the humidity of the gases can change considerable and given that H2O molecules will replace (or be replace by) N2 and O2 molecules, and due to the difference in the molar masses of H2O and the other two gases, the molar mass of the mixture (air) will change considerably and this will cause changes in the atmosphere pressure.