Power units derive from energy per unit time

Steps:

• Power is the rate of energy dissipation, so its unit is energy divided by time.
• Energy dissipated in a resistor equals work done, measured in joules (J).
• One joule is one newton-meter (N m), the unit of work or energy.
• Dividing by time in seconds gives N m s^{-1}, matching watt (W).

Why D is correct:

• Power P = energy/time, and since energy = force × distance = N m, P = N m s^{-1} by definition of the watt.

Why the others are wrong:

• A: N s^{-1} represents force per time, akin to a rate of momentum change, not energy rate.
• B: N s is the unit of impulse or momentum, not involving energy or time rate.
• C: N m is the unit of energy or torque, lacking the per-second factor for power.

Final answer: D