Consider a photon of continuous X-ray coming from a Coolidge tube. Its energy comes from
The kinetic energy of the striking electron
Let's look at the options.
One of it says that the x ray photon gets its energy from atomic transition.
Sounds possible right. In previous chapter we read about hydrogen emission spectrum that happened also because of atomic transition. But this is not how at least continuous x rays are produced.
Another options mentions that the photon gets its energy from the kinetic energy of ions.
Again we didn't go through any such process of continuous x ray's production.
One of the option says that the free electron's energy in the target is converted into x ray.
Confused????
Let's just recall how it all happens.
We heat the coil by passing current and thus the free electrons in the coil are ejected out. "Thermionic emission".
They are accelerated towards the target plate by high potential difference thereby gaining a very high energy. Now when they hit the target they are stopped with huge deceleration, and by now I think you know pretty well that Maxwell's law predicted that any accelerating charge will radiate energy. So technically the kinetic energy of the striking electron is the source for the energy of continuous x ray produced.