Which could be the missing term of the geometric sequence 5, __, 125, ... ?

This question is asking you to find the geometric mean.

The equation for the geometric mean is:

(a*b*c etc) ^ (1/n) where a, b, c are elements in the sequence and n is the number of elements ... in this case the geometric mean is:

(5*125) ^ (1/2) which is the same as

√625

25