What can happen when an insert enters the lacZ gene in blue-white screening?

Correct answer: lacZ can become inactive

In blue-white screening, the cloning site is commonly positioned within the lacZ coding region of a suitable plasmid. If a foreign DNA insert is ligated into that site, it can interrupt lacZ, a process called insertional inactivation. In bacteria supplied with the appropriate substrate, cells with an intact lacZ function generally form blue colonies, whereas cells in which the gene is disrupted by an insert generally form white colonies. Thus option A is correct. The result is not an absolutely universal statement about every system because expression conditions and host genotype matter, but disruption of lacZ is the intended screening principle. The other options describe no biological consequence of insertion.