Answer: Option C
$$\eqalign{
& {\text{Let}}, \cr
& \sqrt {\frac{{16}}{{25}}} \times \sqrt {\frac{x}{{25}}} \times \frac{{16}}{{25}} = \frac{{256}}{{625}} \cr
& {\text{Then}}, \cr
& \frac{4}{5} \times \frac{{\sqrt x }}{5} \times \frac{{16}}{{25}} = \frac{{256}}{{625}} \cr
& \Leftrightarrow \frac{{64\sqrt x }}{{625}} = \frac{{256}}{{625}} \cr
& \Leftrightarrow \sqrt x = \frac{{256}}{{625}} \times \frac{{625}}{{64}} \cr
& \Leftrightarrow x = {4^2} \cr
& \Leftrightarrow x = 16 \cr} $$