Answer:
a. repressor
b. regulator
c. operator
Step-by-step explanation:
All constituent genes of an operon are transcribed in a coordinated manner, such as polycistronic mRNA, that is, multigenic, which is sequentially translated into proteins by ribosomes. The initiation of transcription can be regulated positively or negatively. The genes under negative control are constantly expressed unless they are "disconnected" by a repressor protein that will prevent gene expression by binding to a specific DNA sequence called operator, preventing RNA polymerase from initiating transcription in the promoter.
Those genes whose expression is under positive control will not be transcribed unless an activating protein is present which binds to a specific DNA sequence and helps RNA polymerase in the initial steps of transcription.