1625 × ?25 × 1625 = 256625

Answer: Option C

$$\begin{array}{rl}& \text{Let},\\ & \sqrt{\frac{16}{25}}\times \sqrt{\frac{x}{25}}\times \frac{16}{25}=\frac{256}{625}\\ & \text{Then},\\ & \frac{4}{5}\times \frac{\sqrt{x}}{5}\times \frac{16}{25}=\frac{256}{625}\\ & \iff \frac{64\sqrt{x}}{625}=\frac{256}{625}\\ & \iff \sqrt{x}=\frac{256}{625}\times \frac{625}{64}\\ & \iff x=4^2\\ & \iff x=16\end{array}$$