Equivalent Units for Potential Difference Steps:

• Potential difference is defined as energy per unit charge, with SI unit volt (V) = 1 J/C.
• A (V) matches the standard SI unit directly.
• B (N m C^{-1}) = (N·m)/C = J/C = V, since 1 J = 1 N·m.
• C (W A^{-1}) = (J/s)/A = J/(A·s) = J/C = V, since 1 C = 1 A·s.
• D (J V^{-1}) = J/V = J/(J/C) = C, unit of charge.

Why D is correct:

• J V^{-1} equals the coulomb (C), unit of electric charge, not potential difference (from V = J/C).

Why the others are wrong:

• A: V is the defined SI unit for potential difference.
• B: N m C^{-1} derives from J/C via work-energy equivalence.
• C: W A^{-1} derives from power-current relation as J/C.

Final answer: D