The correct option is (a) 60
Explanation: EEG machines have a notch filter sharply tuned at 60 Hz so as to eliminate mains frequency interference. The use of notch filters should preferably be restricted to exceptional circumstances when all other methods of eliminating interference have been found to be ineffective. These however have the undesirable property of ‘ringing’ i.e. they produce a damped oscillatory response to a square wave calibration waveform or a muscle potential.
