Answer
(a) Less than
(b) II
Work Step by Step
(a) We know that proton 1 is moving at a right angle to the magnetic field and hence the force experienced by proton 1 is $F_{pronton1}=qvBsin90^{\circ}=qvB(because \space sin90^{\circ}=1)$. Similarly, proton 2 is moving parallel to the magnetic field direction and hence the force experienced by proton 2 is given as $F_{proton2}=qvBsin0^{\circ}=0 (because\space sin0^{\circ}=0)$
Thus, the magnitude of force experienced by proton 2 is less than the magnitude experienced by proton 1.
(b) We know that the best explanation is option (II) -- that is, Proton 1 experiences a greater force because it moves at right angles to the magnetic field.
