A particle having charge $q$ enters a region of uniform magnetic field $B$ directed inwards and is deflected as shown to a distance $d$ above the original line of flight as shown. What is the (magnitude of) momentum of the particle?

Question

$\begin{array}{1 1} (A) \large\frac{qBd}{2} \\ (B) \large\frac{qBa}{2} \\ (C) \large\frac{qBa^2}{2d} \\ (D) \large\frac{qB}{2} \bigg( \normalsize d + \large\frac{a^2}{d} \bigg)\end{array}$