Which of the following is a unit of energy ?

Options

A. J/s
B. Watt-day
C. g `cm//s^(2)`
D. kilowatt

Answer

To determine which of the following options is a unit of energy, let's analyze each option provided:

1. **Joule per second (J/s)**:
- This is a unit of power, not energy. Power is defined as the rate at which work is done or energy is transferred. Thus, J/s = Watt.

2. **Watt day (W·d)**:
- A watt is a unit of power (1 Watt = 1 Joule/second). When we multiply watts by time (in days), we get energy. Specifically, 1 Watt day = 1 Watt × 86400 seconds (since there are 86400 seconds in a day) = 86400 Joules. Therefore, Watt day is indeed a unit of energy.

3. **g centimeter per second square (g·cm/s²)**:
- This is not a standard unit of energy. It represents a force (mass times acceleration) and does not directly relate to energy.

4. **Kilowatt (kW)**:
- This is also a unit of power. 1 Kilowatt = 1000 Watts, which again is a measure of energy transfer per unit time.

Based on this analysis, the only option that represents a unit of energy is **Watt day**.

### Final Answer:
**Watt day (W·d)** is a unit of energy.