The diagram shows an experiment set up to investigate the photoelectric effect - Scottish Highers Physics - Question 15 - 2019

To determine the relationship between the current (I) in the circuit and the frequency (f) of the incident radiation, we must analyze the photoelectric effect. As the frequency of the incident radiation increases, the photoelectric current initially increases but will eventually plateau after reaching a certain frequency threshold (the threshold frequency). Graph E accurately displays this relationship:

1. At lower frequencies, the current is zero as the energy of the incident photons is not enough to liberate electrons from the zinc plate.
2. Once the frequency exceeds the threshold, there is a linear relationship where increasing frequency increases the current, provided that the intensity of the radiation remains constant.
3. After reaching a point, the current will stabilize regardless of further increases in frequency.

Hence, the correct answer is graph E.