Metformin is a drug used to treat type 2 diabetes by decreasing glucose production in the liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of glucose metabolism. Metformin activates AMPK in liver cells but cannot cross the plasma membrane. By blocking AMPK with an inhibitor, researchers found that AMPK activation is required for metformin to produce an inhibitory effect on glucose production by liver cells.
Which of the following best describes the component that metformin represents in a signal transduction pathway that regulates glucose production in the liver?

a. It is a secondary messenger that amplifies a signal through a cascade reaction.
b. It is a ligand that activates the signal transduction pathway of the activation of AMPK.
c. It is an allosteric regulator that binds to a crucial section of the DNA that makes the enzymes needed for glucose uptake.
d. It is a competitive inhibitor that binds to glucose and prevents it from entering the cell.