Question

When an electron strikes the target in a Coolidge tube, its entire kinetic energy (a) is converted into a photon

Answer 1

May be converted into a photon.
May be converted into heat.

When an electron strikes the target in a Coolidge tube, the kinetic energy of the electron is used in two ways. Some part of the kinetic energy is converted into a photon, while the remaining part gets converted into heat when the electron makes collisions with the atoms of the target. However, the amount of kinetic energy appearing as the photon vary from collision to collision.