Explanation:
Algae and other submerged plants exhibit a phenomenon called diurnal vertical migration, where they rise to the surface of the water during the day and sink to the bottom at night. The correct answer to why this occurs is option 'c', which states that they become buoyant due to the accumulation of oxygen (O2) as a result of photosynthesis.

Photosynthesis and Oxygen Production:
During daylight hours, algae and other submerged plants conduct photosynthesis, a process in which they use sunlight, carbon dioxide (CO2), and water (H2O) to produce glucose (food) and oxygen (O2). This process occurs in specialized organelles called chloroplasts, which contain the pigment chlorophyll.

Accumulation of Oxygen:
As these plants photosynthesize, they release oxygen into the surrounding water. The oxygen produced is then dissolved in the water, causing an increase in oxygen concentration. This accumulation of oxygen in the water leads to the plants becoming buoyant, making them rise to the surface.

Role of Oxygen in Buoyancy:
The accumulation of oxygen in the water reduces its density, making it less dense than the plants themselves. As a result, the plants experience a net upward force, causing them to float or remain suspended near the water's surface. This buoyancy enables them to maximize their exposure to sunlight, which is essential for photosynthesis.

Sinking at Night:
At night, when there is no sunlight available for photosynthesis, the plants cease their oxygen production. As a result, the oxygen concentration in the water decreases, and the water becomes denser. This increase in density causes the plants to lose their buoyancy and sink down to the bottom of the water body.

Significance of Diurnal Vertical Migration:
The diurnal vertical migration exhibited by algae and other submerged plants serves several purposes. By rising to the surface during the day, they can access sunlight for photosynthesis, ensuring the production of food and energy. Additionally, being near the surface allows them to disperse their offspring or spores effectively. Sinking to the bottom at night provides protection from herbivores and other potential threats present near the surface.

In conclusion, algae and other submerged plants rise to the surface of the water during the day and sink at night due to the accumulation and reduction of oxygen, respectively. This diurnal vertical migration enables them to optimize their photosynthesis, reproductive strategies, and protection in their aquatic environment.