Final answer:
The lac operon is an inducible system in E. coli that is usually off and is turned on by an inducer, allolactose, when lactose is present in the environment(OPTION A).
Step-by-step explanation:
The lac operon is an exemplary model in Biology to understand gene regulation in prokaryotes. It is an inducible system, which means that it is usually off and is turned on by an inducer(Option A).
Specifically, when lactose is present in the environment of E. coli, it's converted into allolactose by the enzyme beta-galactosidase. Allolactose then acts as an inducer that binds to the repressor protein, preventing the repressor from binding to the operator sequence. This allows RNA polymerase to access the DNA and begin transcription of the genes necessary for lactose metabolism. Hence, the correct answer to the student's question is A) usually off and turned on by the inducer. This regulation ensures the efficient use of resources within the cell and only expresses genes for lactose metabolism when needed.