(b) According to the following figure, $\sin\theta = \frac{d}{r}$
Also $r = \frac{\sqrt{2mk}}{qB} = \frac{1}{B}\sqrt{\frac{2mV}{q}}$
$\therefore\sin\theta = Bd\sqrt{\frac{q}{2mV}}$
$= 0.51 \times 0.1\sqrt{\frac{1.6 \times 10^{- 19}}{2 \times 1.67 \times 10^{- 27} \times 500 \times 10^{3}}}$
$= \frac{1}{2} \Rightarrow \theta = 30$