A radiation is incident on a metal surface of work function 2.3 eV. The wavelength of incident radiation is 600 nm. The number of photoelectrons emitted is (Take hc = 1240 eV nm)

zero

Energy of a photon is

$E=\frac{hc}{λ}=\frac{1240eV\,nm}{600nm}=2.07eV$

Since the incident energy (E = 2.07 eV) is less than work function ($\phi_0$ = 2.3 eV) no photoelectrons are emitted.