Final answer:
Water is the electron donor for photosynthesis, and oxygen is the byproduct when water gives up its electrons. Hence, a) is correct.
Step-by-step explanation:
The electron donor for all of photosynthesis is water. During the light reaction of photosynthesis, water molecules are split, releasing electrons that are used to generate energy.
The byproduct that is created when water gives up its electrons is oxygen. This oxygen is then released into the atmosphere as a waste product of photosynthesis.