This is a common factor, but not the greatest common factor. To find the GCF of two monomials, factor each into primes and variables. Find each prime they have in common and each variable they have in common. Make sure that you count how many times they have that factor in common. The GCF should have that many. The correct answer is $\displaystyle{8}{{m}}^{{3}}{{n}}^{{3}}\usepackage{cmbright}\Huge\$ .