In regulation by repressiona. a sugar, such as lactose, acts as an inducer and combines with the repressor to prevent transcription. b. an amino acid activates the repressor so that the repressor binds to the operator and prevents transcription. c. an inducer activates the activator so that it binds to DNA and prevents transcription. d. an amino acid binds to the operator, blocking the repressor, allowing transcription to proceed.

In regulation by repression "an amino acid activates the repressor so that the repressor binds to the operator and prevents transcription".

Option: B

Explanation:

A repressor refers RNA- or DNA inbuilt protein which impedes more than one gene transcription when inbuilt to an operator or related silencer in molecular genetics. A DNA-binding repressor restricts the RNA polymerase linkage to the promoter, stopping the genes from staying transcribed into messenger(mRNA).

An RNA-binding repressor binds to mRNA and prevents the mRNA from being converted into protein. This blockage of speech is known as repression. In this, a protein entity called a repressor inhibits the production of an enzyme by binding and thus blocking the function of deoxyribonucleic acid, which regulates the mechanism by which the enzyme is synthesized.