Final answer:
A repressible operon is one whose transcription is repressed by effector molecules. The presence of the effector molecule causes the repressor to bind to the operator, blocking transcription of the operon.
Step-by-step explanation:
An operon that is repressed by effector molecules is called a repressible operon. These operons typically contain genes encoding enzymes required for a biosynthetic pathway. When the product of the pathway accumulates, it acts as an effector molecule and binds to a repressor, thereby preventing further transcription of the operon.