Electromotive Force Definition Steps:

• Define emf as the maximum potential difference a cell can provide, equal to energy supplied per unit charge by non-electrostatic forces.
• Compare option A: matches definition of energy per unit charge provided by the cell.
• Compare option B: describes internal energy dissipation, not emf.
• Compare option C: refers to voltage drop across internal resistance (Ir), not emf.
• Select A as it aligns precisely with the standard definition ε = W/q.

Why A is correct:

• Emf is defined as the work done by the cell's chemical reactions per unit charge (ε = W/q), representing energy provided to the circuit.

Why the others are wrong:

• B: Describes energy losses (e.g., heat) in the cell due to internal resistance, not the emf provided.
• C: Potential difference across internal resistance is Ir, a drop subtracted from emf.
• D: Terminal potential difference is emf minus Ir when current flows, not the emf itself.

Final answer: A