Which System Is Most Likely to Produce the Stimulus-Response Curve

Which system is most likely to produce the stimulus-response curve shown here?
In the systems shown below,the concentration of molecule 3 represents the stimulus,and the concentration of molecule 2 is the response.Molecule 1 is a substrate of molecule 3.Molecule 4 is an inactive form of molecule 3.Molecule 2 activates molecule 5.

A)

B)

C)

D)

E)