Practice question

Pr and Pfr (B) is correct here. This is core PHOTOPERIODISM: once you know the definition or pathway step, Pr and Pfr is the clear fit. The wrong ones are A) P680 and P700; C) PSI and PSII; D) FAD and FMN. If the topic is about gradients or potentials, water/solutes move from higher to lower of the relevant quantity.