Final answer:
The rate-limiting step of cholesterol biosynthesis involves the enzyme HMG-CoA reductase. It is inhibited by glucagon through the process of phosphorylation, not activated by insulin, cortisol, or free cholesterol.
Step-by-step explanation:
The rate-limiting step or committed step of cholesterol biosynthesis is regulated by several factors including hormonal control. This crucial step in cholesterol synthesis involves the enzyme HMG-CoA reductase, which is regulated through feedback inhibition by cholesterol and its intermediate, mevalonate. Furthermore, HMG-CoA reductase can be phosphorylated and hence inactivated by glucagon and epinephrine. Conversely, it can be dephosphorylated and activated, which is not directly affected by insulin, cortisol, or free cholesterol. Therefore, the correct answer to the question is that the rate-limiting step of cholesterol biosynthesis is inhibited by glucagon, which implicates the phosphorylation process.