(c) Since specific heat of lead is given in $Joules,$ hence use $W = Q$ instead of $W = JQ.$
$\Rightarrow \frac{1}{2} \times \left( \frac{1}{2}mv^{2} \right) = m.c.\mathrm{\Delta}\theta \Rightarrow \mathrm{\Delta}\theta = \frac{v^{2}}{4c} = \frac{(300)^{2}}{4 \times 150} = 150℃$