A neutron, a proton , and an electron 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 fig. the electron follows track .... and the alpha particle follows track.....

Explanation

We know that force on a charged particle in the magnetic field is
`|vecF|=q|vecvxxvecB|` or `F=qvB sin theta`, so
(i) For neutral particle i.e. neutron, `q=0`, hence, `F=0`. It means neutron will go undeflected i.e., track C corresponds to neutron.
(ii) For negatively charged particle i.e. electron, the direction of force, according to Fleming's Left hand rule will be towards right. So track D corrresponds to electron.
(iii) For positively charged particle, the direction of force, according to Fleming's left hand rule will be towards left. So both tracks A and B correspond to positively charged particles (i.e. protons and `alpha`-particles).
When a moving charged particle is subjected to a perpendicular magnetic field, it describes a circular path of radius r given by
`r=(mv)/(Bq)` or `rpropm/q`
`r_p/r_alpha=(m_p)/(m_alpha)xx(q_alpha)/(q_p)=((m)/(4m))xx((2e)/(e))=1/2`
or `r_alpha=2r_p` i.e. `r_alphagtr_p`.
i.e., track B corresponds to `alpha`-particle and track A to proton.