Explanation

(b) For photoelectronic emission to take place wavelength of incident light must be less that the threshold value which is given as $5200\mathring{\mathrm{A}}.$
The wavelength of infra-red light = $7800\mathring{\mathrm{A}}.$
The wavelength of ultra-violet light = $4000\mathring{\mathrm{A}}$. Thus, it is obvious that wavelength of UV radiation is less than the threshold value. Hence, it can emit photoelectrons from the surface of metal.