For a diatomic gas:
\[
C_P = C_V + R
\]
Since \( C_P = C \), we get:
\[
C_V = C - R
\]
For a monatomic gas:
\[
C_V' = \frac{3}{2} R
\]
Using \( C_P = \frac{7}{2} R \), we write:
\[
C_V' = \frac{3}{7} C
\]
Thus, the correct answer is \( \frac{3C}{7} \).