The oxygen which is released into the air by a plant originates from: the release from carbohydrate decomposition from co2 gas splitting of water molecules protein synthesis

The answer is splitting of water molecules. This happens during the light stage of photosynthesis where light energy from the sun is absorbed by chlorophyll is used to split water molecules into hydrogen ions and oxygen. This process is called photolysis. The hydrogen ions joins the dark stage or the light independent stage, while oxygen gas is either released to the atmosphere or used for cellular respiration.