Option (B) is correct because the blue-white selection method is based on the expression of the lacZ gene. According to the context, the lacZ gene present in the plasmid vector expresses the β-galactosidase enzyme. When this gene remains intact and functional (non-recombinant plasmid), the enzyme is produced, which cleaves the colourless substrate X-gal to form a blue pigment, resulting in blue colonies. When the lacZ gene is inactivated due to insertion of foreign DNA (recombinant plasmid), the enzyme is not expressed, and white colonies are formed. Thus, the method relies on whether the lacZ gene is expressed or inactivated. Options (A), (C), and (D) are incorrect because: (A) X-gal is a chromogenic substrate, not a staining dye; (C) recombinant colonies are white, not blue; and (D) the foreign DNA is inserted into the lacZ gene, not the other way around.