The correct option is A Oxygen
Photosynthesis is the process in which carbon dioxide, water, and sunlight is used to form their food (sugar) and oxygen is released. It takes place in two phases, light and dark. In light phases, light dependent reaction takes place in the PSII, where energy from sunlight is used to split water, which releases two electrons, two hydrogen atoms and one oxygen atom. The electrons are used to form NADPH and ATP. The light independent reactions take place in the dark phase, which is also called Calvin cycle.