What could lead you to conclude that the lac operon is on?

The X-gal medium is suitable for detection of gene regulation of the lack operon gene. The media turns from white to blue in the presence of beta-galactosidase, a gene product of lacZ. Beta-galactosidase cleaves lactose into galactose and glucose. Utilisation of lactose as an energy source by Escherichia occurs when there shortage of glucose in the environment.

I’d say a : water molecules