When light energy excites electrons in photosystem ii, where do the electrons to replace them come from? . a. atp. b. photosystem i. c. the light energy. d. split water molecules.

2H₂O⇒4e⁻ + 4H⁺ using this equation  the electrons replace the lost ones in chlorophyl photolysis occurs.
photosystem II "splits" water 
To replace its lost electrons 
Produces oxygen
2 H2O 4 H 4 e- O2
This process is called oxygenic photosynthesis