Photoelectrons are emitted by a metal surface only when the light shining on the surface exceeds a certain minimum:
a wavelength
b frequency
c velocity
d charge

Explanation:

Photoelectric effect is described as the phenomenon of emission of electrons when light falls on the metal surface.

The theory assumes particle theory of light.

When photons hit the metal surface,the energy of photon is used to overcome to binding force between electron and nucleus.

The energy of photon is  given by where is placks constant and is frequency.

When the frequency of photon is low,the energy of photon may not be sufficient to emit a electron.

So,emission requires a minimum frequency.