(A) Gamma rays
Focussing is a process in which a beam of light is passed and concentrated on the particular point.
Gamma rays are the type of elecromagnetic radiation that cannot of focused. Gamma rays has high frequency and also are quite energetic due to which when the beam of light is passed through it, it becomes too difficult to focus on a particular point as they interacts strongly with the matter and destroys itself. Hence, Gamma rays are not easily focused.