a
Given, Gaseous hydrogen.
To find, At what temperature root mean square velocity of gaseaus hydrogen is equal to that of oxygen molecule at $47^{\circ} C$
we know, Root mean square velocity
$V=\sqrt{\frac{3 R T}{m}}$
So, Let at temperature T, RMS velocity of hydrogen is equal to oxygen at $47^{\circ} C$.
$\therefore \sqrt{\frac{3 R T}{2}}=\sqrt{\frac{3 R \times(273+47)}{32}}$
$\Rightarrow T=20 \; K$