Step 1: Understanding the Concept:
By the law of equipartition, each quadratic energy term contributes \(\frac12RT\) per mole.
Step 2: Count the terms:
A non-rigid diatomic gas has 3 translational and 2 rotational degrees of freedom. It also vibrates, which adds 2 more terms (kinetic and potential), so the energy has \(3+2+2 = 7\) terms.
\[ U = \frac72RT,\quad C_V = \frac{dU}{dT} = \frac72R \]
Step 3: Other options:
\(\frac52R\) is for a rigid diatomic molecule. \(\frac32R\) is for a monatomic gas.
Final Answer:
\(C_V = \frac72R\), option (D).
\[ \boxed{\frac{7}{2}R} \]