b
In the given circuit $6 \,\Omega$ and $3 \,\Omega$ are in parallel, and hence its equivalent resistance is given by
$\frac{1}{R_{p}}=\frac{1}{6}+\frac{1}{3} \quad \text { or } \quad R_{p}=2 \,\Omega$.
The equivalent circuit diagram is given in figure.
Total current in the circuit,
$I=\frac{18}{2+4}=3\, \mathrm{A}$.
Power in the circuit $ = VI = 18 \times 3 = 54\,\,watt.$
