According to the question,

A=30 km/h, B=15 km/h, C=10 km/h

we know the formula,

Average speed= $\frac{3\mathrm{ABC}}{\text{AB+BC+CD}}$

∴ Required average speed

$$\begin{gathered} = \frac{3\times 30\times 15\times 10}{30\times 15+15\times 10+10\times 30} \\ =\frac{13500}{900} \end{gathered}$$

 = 15 km/h

Hence, Option C is correct.