1. Identify the specific heats at constant volume

For an ideal gas, $C_V = \frac{f}{2}R$ where $f$ is the number of degrees of freedom.

• For one mole of monoatomic gas: $f=3 \implies C_{V,\text{mono}} = \frac{3}{2}R$

• For one mole of rigid diatomic gas at room temperature: A rigid diatomic gas has 5 active degrees of freedom (3 translational + 2 rotational), so $C_{V,\text{dia}} = \frac{5}{2}R$

2. Find the required ratio

The question asks for the ratio of specific heat at constant volume of the gases respectively, i.e. $\frac{C_{V,\text{mono}}}{C_{V,\text{dia}}}$

So, $\frac{C_{V,\text{mono}}}{C_{V,\text{dia}}} = \frac{\frac{3}{2}R}{\frac{5}{2}R} = \frac{3}{5}$

3. Match with the options

$\frac{3}{5}$ corresponds to Option B.

4. Comparison with stored answer

Stored correct answer: B

Our derived answer: B

So, the derived answer agrees with the stored answer.