The EMF of a cell is:
A. The potential difference across its terminals.
B. The potential difference across its terminals when no current is passing through it.
C. The heat produced when the cell is connected across 1 ohm resistor.
D. The total work done per coulomb of electricity in a circuit in which the cell is connected.

Hint: The potential difference applied due to cell and the EMF of a cell is not the same thing. Potential difference is the measure of total energy between any two points. Ideally, potential difference is equal to EMF value. But in practical cases, the cell itself has some resistance, called the internal resistance of the cell. Hence the potential difference of cells is always less than or equal to the EMF value or the EMF is the largest value of potential difference.

Complete answer:
Since the potential difference for a real cell is always less than the EMF value, hence option A. is incorrect.
But if the circuit is opened, no current will flow through it. If there’s no current in the circuit, the voltage will not be affected by resistance as if there’s no current, resistance also can’t be experienced in the circuit. Hence in open circuit, potential difference and EMF are the same thing. Hence, option B. is correct.
Again, heat produced when connected across 1 ohm resistor is the energy and both energy and EMF are two different physical quantities.
For work done per unit charge (or coulomb), by definition, it means the potential of a region. This is irrespective of the resistance.

So, the correct answer is “Option D”.

Note:
Since the potential difference for a real cell is always less than the EMF value, hence option A. is incorrect.
But if the circuit is opened, no current will flow through it. If there’s no current in the circuit, the voltage will not be affected by resistance as if there’s no current, resistance also can’t be experienced in the circuit. Hence in open circuit, potential difference and EMF are the same thing. Hence, option B. is correct.
Again, heat produced when connected across 1 ohm resistor is the energy and both energy and EMF are two different physical quantities.
For work done per unit charge (or coulomb), by definition, it means the potential of a region. This is irrespective of the resistance, and hence option D. is also correct.