This will be prevented if the activator fastens to the repressor

The correct option is Binding of the repressor to the operator.

In gene regulation, particularly in bacterial operons, the operator is a DNA sequence where a transcription factor (the repressor) binds to prevent the transcription of downstream genes. The repressor physically blocks RNA polymerase from moving along the DNA strand.

If an activator protein binds directly to the repressor and fastens it in an inactive conformation or prevents its interaction with DNA, the repressor will no longer be able to attach to its target sequence. Therefore, this interaction specifically prevents the binding of the repressor to the operator, allowing transcription to proceed.