the frequency of wave will be same as that of cork because the maximum upward displacement of cork will correspond to the crest of the wave and maximum downward displacement of the cork will correspond to trough of the wave thus one to and fro motion of the cork will correspond to one wave and as the cork go through v to and fro motions in one second v nu of times the water will go up and down in one second and hence the frequency of the water will be v only.