Based on the scenario being described within the question it can be said that the form of regulation that is being described is known as attenuation. In the context of genetics, this is a specific mechanism that provides control in various bacterial operons that result in term results in premature termination of the transcription.
I'm going to say since I cannot see the graph i'm guessing that it shows an enzyme at its peak and lowering so i'm going to say the first one 1) There is a specific pH range in which the enzyme effectively lowers the reaction's activation energy
