A neutron, a proton and an $\alpha$-particle enter a region of constant magnetic field with equal velocities. The magnetic field is along the inward normal to the plane of the paper. The tracks of the particles are labelled in figure. The electron follows tracks …… and the $\alpha$-particle follows track…….

Path $C$ is undeviated. Therefore, it is of neutron's path. From Fleming's left hand rule magnetic force on positive charge will be leftwards and on negative charge is rightwards. Therefore, track $D$ is of electron. Among $A$ and $B$ on one is of proton and the other is of $\alpha$-particle. Further,

$r=\frac{mv}{Bq}$ or $r\propto \frac{m}{q}$

Since,

 $$\begin{gathered} {\left(\frac{m}{q}\right)}_{\alpha }>{\left(\frac{m}{q}\right)}_p \\ \therefore r_{\alpha }>r_p \end{gathered}$$

or track $B$ is of $\alpha$-particle.