Cv for an ideal gas

Answer: Option D

For a real gas the $$C_V$$ has an weak relationship with volume and pressure in many cases this relation can be neglected and depends only on temperature whereas for ideal gases $$C_V$$ is solely dependent on temperature.
The usually relationship is given by $$C_V=a+bT+C{T}^2+...$$     and these constants $$a,b,c\dots$$ varies from gas to gas.