Electron emission from a metallic surface by application of light is known as
Electron emission from a metallic surface by the application of light is known as photoelectric emission. When a beam of light strikes the surface of cathode the energy of photons of light is transfer to the free electrons of cathode.
In thermionic emission, the electron emission is achieved by heating the electrode. Due to heating the electrons get enough energy that they emit from the surface of that material.
Electron experience a force when it is in an electrostatic field. When metal surface is placed in an electric field the electron rotating in their orbits experience a force. An electron is attracted towards the positive electrode and experience a force in the direction opposite to that field. Hence the process of electron emission by application of strong electric field at the surface of a metal is called field emission.
The correct option is B.