The correct option is C The difference between the reduction potential of cathode and anode.
The electromotive force (EMF) is the maximum potential difference between two electrodes of a galvanic/voltaic cell. This quantity is related to the tendency for an element, a compound or an ion to acquire (i.e. gain) or release (i.e. lose) electrons. In spite of its slightly misleading name, the EMF is actually a not a force. In general, it is measured in volts.