The correct answer is option 1: Photosynthesis. Photosynthesis is the process by which plants, algae, and some bacteria convert sunlight, carbon dioxide, and water into glucose (a form of sugar) and oxygen. During this process, light energy is used to split water molecules into hydrogen and oxygen, with the oxygen being released into the atmosphere. This is the primary source of oxygen in the Earth`s atmosphere, accounting for approximately 70% of the oxygen produced.

Option 2, excretion, is incorrect as excretion refers to the elimination of waste products from an organism`s body and does not involve the production of oxygen.

Option 3, nitrogen fixation, is also incorrect as nitrogen fixation is the conversion of atmospheric nitrogen into a usable form by certain bacteria and does not directly result in the production of oxygen.

Option 4, respiration, is incorrect as respiration is the process by which organisms release energy from glucose and oxygen, and while it consumes oxygen, it does not produce it.