c
In steady state energy absorbed by middle plate is equal to energy released by middle plate.
$\sigma A (3 T )^4-\sigma A \left( T ^{\prime}\right)^4 $$ =\sigma A \left( T ^{\prime}\right)^4-\sigma A (2 T )^4 $
$(3 T )^4-\left( T ^{\prime}\right)^4 $$ =\left( T ^{\prime}\right)^4-(2 T )^4 $
$\left(2 T ^{\prime}\right)^4 $$ =(16+81) T ^4 $
$T ^{\prime} $$ =\left(\frac{97}{2}\right)^{1 / 4} T$
