Answer :
The condition at which high levels of transcription of structural genes occur in an inducible operon starts when the pathway's substrate is present.
Prokaryotic operons are frequently regulated by repressors binding to operator regions, blocking structural gene transcription.
These operons are characterized as either repressible or inducible. Repressible operons, like as the tryptophan (trp) operon, often comprise genes that encode enzymes necessary for a metabolic process.
A repressible operon will continue to be expressed as long as the pathway's product, such as tryptophan, is required by the cell. When the product of the biosynthetic pathway begins to accumulate in the cell, the operon's expression is suppressed since the cell no longer needs to generate more.
In contrast, inducible operons, such as E. coli's lac operon, frequently contain genes producing enzymes in a pathway involved in the metabolism of a particular substrate, such as lactose. Because these enzymes are only needed when the substrate is present, operon expression is normally activated only when the substrate is present.
The correct answer is option B.
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