b
\(X-\)rays emission: They are due to transitions is the inner energy levels of the atom

Photoelectric emission : Emission of electrons from the metal surface on irradiation with radiation of surface frequency.

Secondary emission: When an electron strikes the surface of a metallic plate, it emits other electrons from the surface.

Thermionic emission: When a metal is heated to a high temperature, the free electron gain kinetic energy and escape from the surface of the metal.