To which of the following quantities, the radius of the circular path of a charged particle moving at right angles to a uniform magnetic field is directly proportional?

If velocity of particle, $\mathrm{v}$ is perpendicular to magnetic field, $\mathbf{B}$ i.e. $\theta=90^{\circ},$ then particle will experience maximum magnetic force, i.e. $F_{\max }=q v B .$ This force acts in a direction perpendicular to the motion of charged particle. Therefore the trajectory of the particle is a circle. In this case path of charged particle is circular and magnetic force provides the necessary centripetal force, i.e. $$ q v B=\frac{m v^{2}}{r} $$ $\Rightarrow$ Radius of path, $r=\frac{m v}{q B}$ $$ r=\frac{p}{q B} \quad[\because p=m v] $$ where, $p=$ momentum of the particle $\therefore r$ momentum Hence, option (d) is correct