The threshold frequency is the frequency below which : A. Photo current increases with voltage B. Photo current decreases with voltage C. Photo electric emission is not possible D. Photo current is constant

The threshold frequency is the minimum frequency of light required to cause the emission of electrons from a metal surface. If the frequency of light is below the threshold frequency, no electrons will be emitted, regardless of the intensity of the light.

Option A is incorrect because photo current increases with voltage only when the frequency of light is above the threshold frequency.

Option B is incorrect because photo current decreases with voltage only when the intensity of light is above a certain level.

Option D is incorrect because photo current is not constant, but increases with increasing light intensity.