For photoelectric emission, the threshold frequency is the

The threshold frequency in photoelectric emission is the minimum frequency of incident radiation required to eject electrons. This means that if the frequency of the incident radiation is below the threshold frequency, no electrons will be emitted, regardless of the intensity or duration of the radiation. However, if the frequency of the incident radiation is equal to or greater than the threshold frequency, electrons will be emitted, and their kinetic energy will increase with increasing frequency. Therefore, the correct option is "minimum frequency of incident radiation required to eject electrons."