Question

When does the lac operon enable a bacterium to build the proteins needed for lactose metabolism only?

Answer 1

When glucose is not available AND when lactose is available

Explanation:

In the absence of lactose, the lac repressor halts production

In the presence of glucose, the catabolite activator protein (CAP), required for production of the enzymes, remains inactive, and EIIAGlc shuts down lactose permease to prevent transport of lactose into the cell.

Source:
https://en.wikipedia.org/wiki/Lac_operon#Regulation

http://www.phschool.com/science/biology_place/biocoach/lacoperon/regulate.html