The correct option is A Dichlorophenolindophenol (DCPIP)
R. Hill described evolution of oxygen during the light reaction of photosynthesis. The path of electrons in light reaction has been elucidated in the Z scheme. It is named so because of its shape and was first proposed by Hill and Bendall, 1960. The illuminated PSII can carry out Hill reaction in the presence of Hill reagents, like ferricyanides and dichlorophenolindophenol (DCPIP).