Final answer:
The presence of glucose decreases cAMP levels, preventing its binding to CAP, which results in decreased transcription of the lac operon.
Step-by-step explanation:
When an E. coli cell is exposed to glucose, this affects the regulation of the lac operon via catabolite activator protein (CAP). The presence of glucose decreases the levels of cyclic AMP (cAMP), which is necessary for CAP to activate transcription of the lac operon. Therefore, the correct answer to the student's question is D) cAMP does not bind to CAP and transcription is decreased. When glucose is plentiful, cAMP levels are low, preventing it from binding to CAP, which keeps CAP in an inactive state. As a result, CAP cannot bind to the promoter region upstream of the lac operon, resulting in decreased transcription of the genes required to metabolize lactose.