The correct option is
C Rate of heart beat
Pacemaker is an electrically-charged device which is placed in the chest region to control abnormal heart rhythms. It is connected to the heart by various wires. It uses electrical pulses in order to signal the heart to beat at a normal rate. It influences the rate of heart rate. It is used in the treatment of arrhythmias.
Thus, the correct answer is 'Rate of heart beat.'