Final answer:
When lactose is not present, E. coli does not express the genes for lactose-digesting enzymes due to the action of a repressor protein binding to the operator, which prevents gene transcription.
Step-by-step explanation:
When lactose is not present in the environment of E. coli, the bacteria do not need the enzymes to break down lactose. Hence, the correct answer is b. The genes that produce the enzymes needed to break down lactose are not expressed. This is because a repressor protein which is produced by the I gene binds tightly to the operator DNA, blocking RNA polymerase from transcribing the genes in the lac operon responsible for lactose digestion. When lactose is present, it binds to the repressor, causing it to release from the operator, allowing transcription of the lac genes and consequent production of lactose-digesting enzymes.