Hello there! :)
In your question, it is asking you to find the equivalent fraction for 1/4. In order to do that, we have to find another denominator that is "compatible" with dividing by the 4 (this number is 8, which is already given to you).
4 goes into 8 twice, so we have to do the same as well and multiply both sides by 2, leaving our denominator as (1)(2)=2.
Therefore, 1/4=2/8. I hope I helped :)