**Solution**:

Let the number be $\dot{x}$.

$$

\begin{aligned}

\therefore \quad & \frac{5}{7} \text { of } x=15 \\

\therefore \quad & x=\frac{15 \times 7}{5}=21 \\

& \text { Again, } \frac{2}{3} \text { of } 21=2 \times 7=14 \\

\therefore \quad & \text { True option }=\text { (d). }

\end{aligned}

$$