What is the S.I. unit of power?

The watt $\left(\mathrm{W}\right)$ is the SI unit of electrical power.
Electric power is the rate at which electrical energy is lost or used up in an electric circuit.
The formula $\mathrm{P}=\mathrm{VI}$ yields the power $\mathrm{P}$.
The watt is the SI unit for electrical power (W). It is the amount of power used by a machine that operates at a potential difference of $1 \mathrm{V}$ and carries $1 \mathrm{A}$ of current.

$Power = \frac{Work Done}{Time}$

SI unit of energy or Work done is Joule (J) and that of time is seconds (S).

Thus, SI unit of power is J/s OR watt (W). 

The S.I Unit of Power is W(watt).

Hence, option A is correct.