Final answer:
In photosynthesis within chloroplasts, water is the molecule that donates electrons after being split by the oxygen-evolving complex, following the excitation of chlorophyll by light energy.
Step-by-step explanation:
In plant chloroplasts, the molecules that serve as the weak electron donor are molecules of water (b). During photosynthesis, sunlight is absorbed by chlorophyll molecules in the photosystems, which become excited and pass the energy to the primary electron acceptor. This energy is then used to extract electrons from water through a process that involves the oxygen-evolving complex. This complex uses the power of the oxidized chlorophyll to oxidize manganese atoms, ultimately making it strong enough to split water molecules, release electrons, four protons (H+), and molecular oxygen (O2).