Fill in the missing number.

$625,5,125,25,25,$ _______ $,5$

The correct option is B $125$
There are two series.

One is $625,125,25,5$ from left to right $\frac{625}{5}=125$

$\frac{125}{5}=25$
$\frac{25}{5}=5$
Another is $5,25,125$ from right to left $5\times 5=25$
$25\times 5=125$
Therefore, $125$ is the answer.
Hence, option 'B' is correct.