If velocity of particle, v is perpendicular to magnetic field, B i.e.
θ=90∘, then particle will experience maximum magnetic force, i.e.
Fmax=qvB. 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.,
qvB= ⇒ Radius of path,
r= r= [∵p=mv] where,
p=p = momentum of the particle
∴r∝ momentum
Hence, option (d) is correct.