Step 1: Understand specific heat at constant volume.
For a monatomic ideal gas, the specific heat at constant volume is given by \( C_V = \frac{3}{2} R \), where \( R \) is the gas constant.
Step 2: Analyze the options.
Thus, the correct answer is \( \frac{5}{3} R \).
Final Answer:
\[
\boxed{\frac{5}{3}R}
\]