The correct option is D insertional inactivation of β-galactosidase in recombinant bacteria
For screening the clones containing recombinant DNA, a chromogenic substrate known as X-gal is added to the agar plate.
If β-galactosidase is produced, X-gal is hydrolyzed to form 5-bromo-4-chloro-indoxyl, which spontaneously dimerizes to produce an insoluble blue pigment called 5,5’-dibromo-4,4’-dichloro-indigo. The colonies formed by non-recombinant cells, therefore appear blue in color while the recombinant ones appear white. The desired recombinant colonies can be easily picked and cultured.