When you see a fraction as the power like this you want to try to simplify it to things that you can do (like power of 1/2 or 2). 125 is one of the key numbers to remember when preparing for your GCSEs becuase it is 5^3. I recommend learning 1-10 cubed so that you recognise it immediatley. It'll make you're life easier!
so for this question we split it into (125^1/3)^2
In general (a^b)^c is a^b x a^b x a^b *...(c times)
And we remember that 125 is 5^3 so this question simplifies to (5)^2
and we know that 5 squared is 25.
So the answer is 25