Plastocyanin acts as an electron carrier. It accepts only one electron from the cytochrome b _6 f complex (and is thus reduced), before reducing PSI (and is thus oxidized). In this step, the electron that is passed to PSI replaces that which has been lost by excitation by a photon of light, thus this may be considered the step that directly links the two photosystems. So, plastocyanin is the immediate electron donor for PS I. Plastoquinone is an electron acceptor which accepts two excited electrons from PSII and 2H ^+ from the stroma to become plastoquinol (PQH _2 ). It has the dual functions of transferring excited electrons from PSII to the cytochrome b6f complex (below) as part of the ETC, and of shuttling protons from the stroma to the thylakoid lumen as part of the generation of a proton motive force. This latter function is important for the generation of ATP. Ferredoxin is the final electron carrier in the light-dependent reactions, which accepts an electron from PSI to become reduced ferredoxin, before reducing NADP ^+ in the reaction catalyzed by ferredoxin NADP ^+ reductase (below). Quinones have important roles in carrying electrons between the components of the light reactions. Since they are lipid soluble, they can diffuse through the membrane. They can carry one or two electrons, and, in their reduced form (with added electrons), they carry hydrogen atoms that can be released as hydrogen ions when the added electrons are passed on, for example, to a cytochrome. So, the correct answer is option D.