The rms velocity of a gas molecule is given by the formula:
\[
v_{\text{rms}} = \sqrt{\frac{3kT}{m}}
\]
where \( k \) is the Boltzmann constant, \( T \) is the temperature, and \( m \) is the mass of the molecule.
Thus, the rms velocity is inversely proportional to the square root of the mass:
\[
v_{\text{rms}} \propto \frac{1}{\sqrt{m}}
\]
So, the correct answer is \( \frac{1}{\sqrt{m}} \).