I'm guessing, since it's a discuss question, that you can draw a conclusion even if it's different to what they're suggesting, and so say that the PE effect shows that light has a particle nature, but that other experiments (eg. cathode rays) show that it also has a wave nature, which has led to the dual nature theory. Hope that helped (hope the picture worked, otherwise find in http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html under wave-particle duality: light). However that is a weird question
I would firstly talk about how wave model successfully predicted that as the intensity of the light increased, photo current should increase also, which it did albeit for a reason other than that predicted by the wave model of light.
I would then go on and talk about 2 things that the wave model could not explain (existence of threshold frequency, instantaneous emission, effect of frequency on Ek of electrons etc..)