b
Heat added at constant volume is equal to the change in internal energy of the system. Change in internal energy $\quad \Delta U=n C_{v} \Delta T$
For $\Delta T=150 K, \quad \Delta U=Q_{v}=6300 \mathrm{J}$
$\therefore \quad 6300=n C_{v}(150)$ $\ldots(1)$
Let the change in internal energy be $\Delta U$ for $\Delta T=300 K$
$\therefore \Delta U=n C_{v}(300)$ $...(2)$
Dividing $( 2 )$ and $( 1 )$ we get $\frac{\Delta U}{6300}=\frac{300}{150}$
$\Longrightarrow \Delta U=12600 \mathrm{J}$