D

Explanation:

1. Concept of Energy and its Units:
Energy is defined as the capacity to do work. The SI unit of energy is the Joule ($ ext{J}$). Energy can also be expressed in terms of Power $ imes$ Time, as $ ext{Power} = \frac{\text{Energy}}{\text{Time}}$.

2. Analysis of Kilowatt-hour ($ ext{kWh}$):
A kilowatt-hour ($ ext{kWh}$) is a commercial unit of energy.
$$\text{1 kWh} = 1 \text{ kW} \times 1 \text{ hour}$$
$$\text{1 kWh} = 1000 \text{ W} \times 3600 \text{ s} = 3.6 \times 10^6 \text{ J}$$
Since it equals a quantity of Joules, kilowatt-hour is a unit of energy.

3. Examination of Other Units:

• Joule-second ($ ext{J}\cdot\text{s}$): Unit of Angular Momentum and Planck's Constant ($h$).
• Watt ($ ext{W}$): Unit of Power ($1 \text{ W} = 1 \text{ J/s}$).
• Newton per meter ($ ext{N/m}$): Unit of Surface Tension and Spring Constant ($k$).

Therefore, the correct unit of energy among the options is Option D (Kilowatt-hour).