The speed of a boat in still water is 15 km/h and the rate of current is 3 km/h. The distance travelled by boat downstream in 12 minutes is

Given,

Boat speed in still water $=15\mathrm{km}/\mathrm{hr}$
Current rate$=3\mathrm{km}/\mathrm{hr}$

Downstream speed$=15+3=18\mathrm{km}/\mathrm{hr}$

So, the time can be given as,

$$\begin{gathered} =12 \min \\ =\frac{12}{60} \mathrm{hr} \\ =\frac{1}{5} \mathrm{hr} \end{gathered}$$

So, the

$$\begin{gathered} \text{ Distance travelled }=\text{ Speed }\times \text{ Time } \\ =18\times \frac{1}{5} \\ =3.6 \mathrm{km} \end{gathered}$$

Hence, option (D) is correct.