Answer:
a. inducible
Step-by-step explanation:
An inducible gene is a gene that is expressed only when its product is needed, and is expressed only in the presence of a substance (an inducer) in the environment. This substance can control the expression of one or more genes (structural genes) involved in the metabolism of that substance. For example, lactose induces the expression of lac genes that are involved in lactose metabolism. And certain antibiotics can induce expression of a gene that leads to resistance to that antibiotic.