C. Binding to the operator.
Explanation:
When the repressor starts binding to the operator, it can not transcribe, or make mRNA. This makes it so the RNA polymerase can't go onto the promoter and bind to it, which is a crucial part in making mRNA. The operator functions by making sure the certain DNA gets made, so it is very important. The lac repressor will turn off the expression of the lac genes by binding to the operator.
