Answer:
The correct answer is B. Repressor.
In prokaryotic cells, an operon is a cluster of genes that are transcribed together as a single unit. The operon consists of a promoter, an operator, and the genes it controls. The operator is a DNA sequence located between the promoter and the genes.
A repressor protein binds to the operator region and prevents RNA polymerase from binding to the promoter and initiating transcription. By binding to the operator, the repressor blocks the expression of the genes in the operon, effectively turning off their transcription.
Therefore, the term that describes a protein that prevents an operon from being turned on in a prokaryotic cell is B. Repressor.
Hope this helps :]