Introduction:
Power is a physical quantity that measures the rate at which work is done or energy is transferred or converted. It is an important concept in physics and engineering. The SI unit of power is the watt (W), named after the Scottish engineer James Watt.

Explanation:
The SI unit of power, the watt, is commonly used to measure the amount of work done per unit time. It is defined as one joule of work per second. Here's a detailed explanation of why the answer is option 'D' - Watt:

1. Definition of Power:
Power is defined as the rate at which work is done or energy is transferred. Mathematically, power is given by the equation:

Power = Work / Time

Where:
- Power is measured in watts (W)
- Work is measured in joules (J)
- Time is measured in seconds (s)

2. Work and Energy:
Work is the amount of energy transferred by a force acting through a distance. It is given by the equation:

Work = Force * Distance * cos(theta)

Where:
- Force is measured in newtons (N)
- Distance is measured in meters (m)
- theta is the angle between the force and the direction of motion

Energy is the capacity to do work. It exists in various forms such as kinetic energy, potential energy, thermal energy, etc.

3. Relationship between Power and Work:
Power is directly proportional to the amount of work done per unit time. In other words, the more work done in a given time, the higher the power.

4. Relationship between Power and Energy:
Since work and energy are related, power can also be defined as the rate at which energy is transferred or converted. This means that the power is the amount of energy transferred or converted per unit time.

5. The SI Unit of Power - Watt:
The SI unit of power is the watt (W), named after James Watt, the Scottish engineer who made significant contributions to the development of the steam engine. One watt is defined as one joule of work or energy transferred per second.

Conclusion:
In conclusion, the SI unit of power is the watt (W). It is defined as one joule of work or energy transferred per second. Power is a fundamental concept in physics and engineering, and it is used to measure the rate at which work is done or energy is transferred or converted.