When ultraviolet rays incident on metal plate then the photoelectric effect does not occur, it occurs by incidence of:

When ultraviolet rays are incident on the metal plate and photoelectric effect does not occur, It means that the threshold frequency is greater than the frequency of ultra-violet rays. X-rays have extremely higher frequencies compared to that of ultra-violet rays. For photoelectric effect, the frequency of the incident light must be greater than the threshold frequency of the metal. Hence, when X-rays are incident, photoelectric emission happens. The correct option is B.