The correct answer is (B) Photosynthesis and diffusion from the air
Dissolved oxygen enters the water from photosynthesis and through air. The The aeration of water can be caused by wind. From the air oxygen diffuse into the water and gets mixed in the it through circulation. The oxygen is also produced as a waste product of photosynthesis from blue-green algae, phytoplankton, etc which is added in water.
