a
(a)
$\because$ Both have same volume
$\therefore a^3=\pi r^2 L \quad \dots (1)$
Amount of radiation $\propto$ Surface area [ $\because$ Temperature, material are same for both]
[ $\because$ Temperature, material are same for both]
$\frac{\text { Radiation cylinder }}{\text { Radiation cube }}=\frac{2 \pi r L}{6 a^2}=\frac{2 \pi r L \cdot a}{6 a^3}$
using equation $(1)$
We get
$\frac{R_{\text {cylinder }}}{R_{\text {cube }}}=\frac{a}{3 r}$