Answer
$a^{3}$
Work Step by Step
$a^{8}=a^{3}\times a^{5}$
$a^{5}=a^{3}\times a^{2}$
$a^{3}$
When all monomials are in terms of $a$ and there are no other variables involved, the GCF is the monomial with the lowest exponent because the other monomials can be divided by it.
