The value of γ=CpCv  for hydrogen, helium and another ideal diatomic gas X (whose molecules are not rigid but have an additional vibrational mode), are respectively equal to,

Diatomic gases have 5 degrees of freedom, if vibrational mode is neglected. And if vibrational mode is also considered then degrees of freedom of diatomic gas molecules are 7.For Hydrogen  γ=1+2f=1+25=75For Helium γ=1+2f=1+23=53For gas X (vibrational mode also considered) f = 7. γ=1+2f=1+27=97  So, 75,53,97