In many cases, the activity of an enzyme is regulated by feedback where the end product acts as a inhibitor.

In many cases, the activity of an enzyme is regulated by Negative feedback where the end product acts as a non competitive inhibitor.
In feedback inhibition the end product of a metabolic pathway acts on the key enzyme regulating entry to that pathway, keeping more of the end product from being produced. The feedback inhibition acts at the first committed step of the pathway, meaning the first step that's effectively irreversible. The pathway steps regulated by feedback inhibition are often catalyzed by allosteric enzymes. 

C- a hypothesis or d- an observation