An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system during motion, one ignores the magnetic force of one on another. This is, because,

1. the two magnetic forces are equal and opposite, so they produce no net effect.

2. the magnetic forces do not work on each particle.

3. the magnetic forces do equal and opposite (but non-zero) work on each panicle.

4. the magnetic forces are necessarily negligible.

(b) Hint: Apply the concept of work.
When electron and proton are moving under influence of their mutual forces, the magnetic will be perpendicular to their motion hence no work is done by these forces.