In a metabolic pathway, a typical control mechanism is to have:

There are choices for this question namely:

a. a reactant inhibit a late step. 
b. the final product inhibit the enzyme responsible for its own production. 
c. a lack of reactant stimulate the pathway. 
d. the final product inhibit an early step.

The correct answer is "the final product inhibit an early step". This is called negative feedback or feedback inhibition. This means that the product of the metabolic pathway will inhibits its own production by inhibiting an early reaction in the pathway.