Three vessels of equal volume contain gases at the same temperature...

The RMS speed is given ${v}_{rms}$ $= \sqrt{\dfrac{3RT}{M}}$.

The temperature and pressure is same for all the gases. So the speed is inversely proportional to the square root of the molar mass of the gases.

${v}_{rms} ( mono ) = \sqrt{\dfrac{3RT}{4 \times {10}^{- 3}}}{v}_{rms} ( dia ) = \sqrt{\dfrac{3RT}{71 \times {10}^{- 3}}}{v}_{rms} ( poly ) = \sqrt{\dfrac{3RT}{352 \times {10}^{- 3}}}$

${v}_{\text{rms}} ∝ \dfrac{1}{\sqrt{M}}$

Hence,

${v}_{rms} ( poly ) < {v}_{rms} ( dia ) < {v}_{rms} ( mono )$.