The correct option is (a) b > R
The explanation: If b > R, it means the interaction parameter is greater than the atomic radius. In this case, the electron is ejected by the photon and it is known as the Photoelectric effect.
If b ≈ R, the incident photons are scattered by the electron of the atom and the electron itself gets scattered. This phenomenon is known as the Compton effect.
If b < R, the photon is directly converted into an electron-positron pair, known as pair production.