Answer :

A fraction is in simplest form when the numerator and denominator share no common factors.

The prime factorization of 5 is 5.
The prime factorization of 6 is 2×3.

The numerator and denominator share no common factors, thus 5/6 is already in simplest form.
5/6 cannot be simplified further because there is no common factor that goes into both the top (numerator) and bottom (denominator) of the fraction.
Hope this helps :)