Insulin is a protein hormone that is secreted in response to elevated blood glucose levels. When insulin binds to its receptors on liver cells, the activated receptors stimulate phosphorylation cascades that cause the translocation of glucose transporters to the plasma membrane. Based on the information provided, which of the following best describes the role of insulin in this liver cell signal transduction pathway\
a. It acts as a ligand.
b. It acts as a receptor.
c. It acts as a secondary messenger.
d. It acts as a protein kinase.