Final answer:
Effervescent tablets go under and then float on the water because the chemical reaction forms gas bubbles that increase buoyancy, causing them to rise to the surface.
Option C is correct.
Step-by-step explanation:
The reason effervescent tablets first go under the water and then float on the water after a while is due to a chemical reaction. When the tablet is placed in water, it dissolves and releases gas bubbles, primarily carbon dioxide (CO₂). This process is known as effervescence. The release of gas creates a fizzing effect, and as the gas bubbles form, they attach to the solid particles of the dissolving tablet, which initially sink due to their higher density compared to water. Over time, the accumulation of gas bubbles increases the buoyancy of the tablet remnants, causing them to rise to the surface. Therefore, the correct answer to the question, 'Why do effervescent tablets go under the water then go on the water after a while?,' is C. Change in buoyancy.