If p1 can't alone invoke p2, why is there an arrow from p1 to p2? Option C says that Tc invokes p1 and then p2. Then the arrow from p1 to p2 is redundant. If Option C were "A new module Tc is defined to control the transaction flow. This module Tc first invokes pl. p1 then invokes p2. p2 in turn invokes p3, or p4, or p5", then its description would match the flow diagram and would be correct. As of now, I think option A is the stronger answer.