Answer:
The correct answer is option b. "repressible gene expression".
Step-by-step explanation:
The trp operon is a classic example of a repressible gene expression. The trp operon controls the biosynthesis of tryptophan from an initial precursor by encoding five genes that translate to three enzymes. These enzymes act as regulators and tryptophan itself acts as the final regulator. Tryptophan inhibits its own production when it binds to the transcription regulator, therefore it is an example of a repressible gene expression.