What occurs when an inducer is added to a medium containing an organism with a metabolic pathway controlled by a repressor?
a. the inducer combines with the repressor and activates the repressor.
b. the inducer combines with the repressor and inactivates the repressor.
c. the inducer combines with the substrate and blocks induction.
d. the inducer combines with the substrate and activates induction.

B. The inducer combines with the repressor and inactivates the repressor.

Explanation:

A repressor is a negative regulatory protein. It inhibits transcription when bound to DNA. The inducer is a molecule that binds to a repressor protein. Binding of inducer to the repressor converts it into an inactive form. The inactive repressor is unable to prevent transcription. Therefore, when the inducer is added to the medium, it would bind to the repressor to inactivate it.

For example, the repressor protein of lac operon binds to the operator region and does not allow the expression of the lac operon. When lactose is present in the medium, few of the lactose molecules enter the bacterial cell and are converted into allolactose which in turn binds to the repressor protein. Binding of allolactose to repressor protein alters the shape of the repressor protein in such a way that it is no longer able to bind to the operator region. Here, allolactose is the inducer.  

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.