The value of $\mathrm{\gamma }\left(=\frac{{\mathbf{C}}_{\mathrm{p}}}{{\mathbf{C}}_{\mathrm{v}}}\right)$  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  $\mathrm{\gamma }=1+\frac{2}{\mathrm{f}}=1+\frac{2}{5}=\frac{7}{5}$

For Helium $\mathrm{\gamma }=1+\frac{2}{\mathrm{f}}=1+\frac{2}{3}=\frac{5}{3}$

For gas X (vibrational mode also considered) f = 7. 

$$\begin{gathered} \mathrm{\gamma }=1+\frac{2}{\mathrm{f}}=1+\frac{2}{7}=\frac{9}{7} \\ \text{ So, }\left(\frac{7}{5},\frac{5}{3},\frac{9}{7}\right) \end{gathered}$$