The lactose analog isopropyl-b-d-thio-galactoside (iptg) is often used to regulate gene expression systems in bacteria. iptg does not act as a substrate for b-galactosidase, but can bind to, and inactivate, the repressor. in this case, iptg serves as a(

a. inducer.
b. repressor.
c. effector.
d. operon.

Option A. inducer.

Explanation:

The lac operon is an inducible operon. The presence of lactose allows the gene expression for the metabolism of lactose. The IPTG mimics the structure of allolactose. The allolactose is the direct inducer of the gene of lac operon. Lactose by converting to allolactose act as a inducer. The repressor in the lac operon binds with the operator region and thus inhibits the process of transcription.

The allolactose binds with repressor and thus inhibits its binding to operator thereby initiating the process of transcription. As the IPTG mimics allolactose so it binds with repressor and act as a inducer. Thus, option A is correct.