Insertional inactivation means inactivation of an enzyme by the presence of an insert (recombinant DNA). In the presence of a chromogenic substrate, active alpha-galactosidase enzyme yields blue coloured colonies (non recombinants). While insertional inactivation of alpha-galactosidase enzyme inactivates the enzyme resulting in white colour colonies (recombinants). Thus, it is due to insertional inactivation of alpha-galactosidase in recombinant bacteria that the colonies of recombinant bacteria appear white in contrast to the blue colonies of non-recombinant bacteria. So, the correct answer is 'Insertional inactivation of alpha galactosidase in recombinant bacteria'.