1. For an ideal gas, the root mean square speed is

$v_{\mathrm{rms}}=\sqrt{\frac{3RT}{M}}$

where:

• $R$ is the gas constant,
• $T$ is the absolute temperature,
• $M$ is the molar mass.

2. Since all three gases are at the same temperature, we get

$v_{\mathrm{rms}} \propto \frac{1}{\sqrt{M}}$

So, the gas with smaller molar mass has greater $v_{\mathrm{rms}}$.

3. Now compare molar masses:

• Neon (monoatomic): $M_{\mathrm{Ne}} \approx 20\,\text{g/mol}$

• Chlorine (diatomic): $M_{\mathrm{Cl_2}} = 2\times 35.5 = 71\,\text{g/mol}$

• Uranium hexafluoride (polyatomic): $M_{\mathrm{UF_6}} = 238 + 6\times 19 = 352\,\text{g/mol}$

4. Therefore,

$M_{\mathrm{Ne}} < M_{\mathrm{Cl_2}} < M_{\mathrm{UF_6}}$

Hence,

$v_{\mathrm{rms}}(\text{mono}) > v_{\mathrm{rms}}(\text{dia}) > v_{\mathrm{rms}}(\text{poly})$

5. Checking options:

• A: All equal — incorrect
• B: $v_{\mathrm{rms}}$(mono) $>$ $v_{\mathrm{rms}}$(dia) $>$ $v_{\mathrm{rms}}$(poly) — correct
• C: Incorrect order
• D: Opposite order — incorrect

Therefore, the correct answer is Option B.