Answer : $\;H_{2}$

Explanation :

$V_{rms} = \sqrt{\large\frac{3RT}{M}}$

or $\;M=\large\frac{3RT}{V_{rms}^{2}}= \large\frac{3 \times 8.31 \times T}{1933 \times 1933}$

$= \large\frac{3 \times 8.31 \times 300 K}{ 1933 \times 1933}$

$= 0.002 Kg$

$M = 2 gms$

Hence $\;H_{2}$