Question

Sol. Key concept: Curvature in a magnetic field identifies charge sign only when the direction is known.

For a charged particle, the magnetic force is q(v × B). With B into the page, positive charges curve counterclockwise. The illustrated track traversed from p to q has this sense of curvature, so a proton or alpha particle can follow it.

Reversing both the direction of motion and the charge leaves the force towards the same curvature centre. Thus a negatively charged electron moving from q to p is also possible. A neutron is uncharged and cannot acquire this curvature from the magnetic field alone.

Accordingly the official-key selection is (a), (b) and (c). The track radius alone does not distinguish proton from alpha particle without momentum information.

Correction / clarification: If the horizontal line represents an absorber and the smaller radius below it indicates energy loss, the travel direction is q to p and only (c) remains. The line is not identified in the stem; the official multiple answer assumes that extra directional information is not imposed.