The lac operon is expressed when .
a. glucose is low and lactose is absent
b. glucose is low and lactose is present
c. glucose is high and lactose is absent
d. glucose is high and lactose is present
e. glucose is low, regardless of the presence or absence of lactose

Option B,  glucose is low and lactose is present

Explanation:]

Lac operon transcription is dependent on two regulatory proteins

•The lac repressor which acts as a  lactose sensor.

•The catabolite activator protein (CAP) which acts as a glucose sensor.

In the presence of glucose, the adenylate cyclase is inhibited due to which cAMP is not produced. But once the glucose level becomes low or negligible, cAMP is produced as adenylate cyclase is not inhibited any more. cAMP produces a complex by combining to catabolite activator protein (CAP), thereby allowing transcription of the lac operon.  

In the absence of lactose, lac repressor binds to operator due to which transcription stops and in case where lactose is present, lac repressor does not binds to operator and hence allows lac transcirption

Hence, option B is correct

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