Count the degrees of freedom f of one molecule of each gas…

(a) Helium. One atom, so it only moves: $f = 3$ and $C_v = \tfrac32 R$.

(b) Carbon dioxide. It is straight, so it turns 2 ways: $f = 3 + 2 = 5$ and $C_v = \tfrac52 R$.

(c) Steam. It is bent, so it turns 3 ways: $f = 3 + 3 = 6$ and $C_v = 3R$.

(d) Hot carbon monoxide. It moves 3 ways, turns 2 ways and vibrates. The vibration has 2 terms: $f = 3 + 2 + 2 = 7$ and $C_v = \tfrac72 R$.