Photosynthesis begins with energy from light being converted to chemical energy. The only answer explanation accurately explaining this process describes the conversion of light energy (photons) to chemical energy (excited electrons).

During the light reactions water is split and the by-product is oxygen. Oxygen is then released back into the atmosphere. Sugars are produced during the Calvin cycle and are not a direct product of the light reactions.

The light reactions are the steps of the photosynthesis that function to convert solar energy into chemical energy that can be used during the Calvin cycle. The light reactions use solar energy to reduce   to  and to convert  to .  

The light reactions of photosynthesis occur in the thylakoid membranes inside of chloroplasts in plant cells. Photosynthesis depends on the buildup of a proton gradient across a membrane to generate ATP. The thylakoid is an organelle present only in plants. The phospholipid bilayer and cell wall surround plant cells, and are not involved in photosynthesis.

The molecule is reduced because when a molecule gains an electron, it is said to be reduced. The answer oxidized would have been chosen if the molecule lost an electron. Plasmolysis has nothing to do with the gain or loss of an electron, and the term redoxed does not fit into this question. Hydrolysis is when a bond is broken using water, so that is incorrect as well.

The light dependent reaction converts solar energy to chemical energy, using the photosystem to capture light energy, and ultimately produce ATP and NADPH. Pigment molecules in the photosystem absorb photons of light, which then reach the chlorophyll, exciting electrons in the chlorophyll. The excited electron then is transferred to a primary electron acceptor of the electron transport chain (and is replaced by the splitting of water (to produce O2, H+ and e-)).

The ATP synthase protein is embedded in the thylakoid membrane. When sunlight is captured by photosystem II in the light dependent reaction, water is split, producing O2 and 2 H+ ions and 2 electrons. As the electrons flow down the electron transport chain, this energy is used to pump hydrogen ions from the stroma into the thylakoid space, producing a high concentration of hydrogen ions inside the thylakoid lumen. Eventually, the hydrogen ions then flow from the area of high concentration (in the thylakoid lumen) to an area of low concentration (in the stroma ), through the ATP synthase protein. The movement of the hydrogen ions through this structure combines ADP with phosphate, producing ATP.

Carbon fixation, catalyzed by Rubisco, occurs during the first step of the light independent reaction.

 ATP and NADPH are produced during the light dependent reaction, and are reactants necessary for the light independent reaction (Calvin Cycle) to occur. During the light independent reaction, ATP and NADPH are used to convert phosphoglycerate molecules (PGA) to glyceraldehyde-3-phosphate molecules (G3P). G3P is then used to produce glucose.

Light dependent reactions convert solar energy to chemical energy (in the form of ATP and NADPH). Sunlight and H2O are the reactant of the light dependent reaction. The sunlight is captured by photosystem II, and is used to excite electrons. Splitting of H2O produces hydrogen ions used to establish the proton gradient, and electrons produced replace the electrons transferred to the primary electron acceptor.