Assume that we have 1 mole of triatomic gas.

Therefore 3NA (Avogadro's Number) is present initially.

So, 0.5 moles have 1.5NA atoms.

Now, 1 part of 0.5 moles remains undissociated.

So. degree of dissociation = 0.5 x 6 = 3

The other 0.5 moles have 1.5NA which breaks into 3 atoms.

So, the total number of atoms = 0.5NA

Now, the degree of freedom for monoatomic gas = 3

So, degree of freedom for 0.5NA monoatomic particles = 1.5 x 0.5 = 0.75

Hence, total will be 3 + 0.75 = 3.75