Genes in an inducible operon are: -

The correct option is B

Unless activated by a signal, they are never expressed.

The correct option is B.

Explanation for the correct option:

1. Inducible operons are the operons that are turned on when the substrate is present which acts as an inducer. In the absence of the inducer, they are turned off.
2. For example, in lac operon, the inducer (lactose) binds with the repressor and prevents the repressor from binding to the operator hence, the genes are expressed.
3. In the absence of the inducer, the repressor binds to the operator and the genes are inaccessible to RNA polymerase for transcription.

Explanation for the incorrect option:

Option A

The inducible operons are not expressed until turned off by a signal instead repressible operons are the ones that are expressed until turned off

Option C

They are not expressed always. They are turned on when required.

Option D

They are expressed when required.