The degrees of freedom of a diatomic gas at normal temperature is

Total Degree of freedom is the sum of translational, rotational and vibrational degree of freedom.

As we know, translational degree of freedom is three as it can move in three axis (x,y and z), rotational degree of freedom is two as it can rotate in two axis (x and y).

As  the vibrational degree of freedom exists at very high temperatures, therefore, for normal temperature it is zero.

f = 3 + 2 = 5

Final Answer: Hence, the correct option is C i.e., 5.