D

Step 1: When water is heated, various physical properties of the water and the air bubble change.
Step 2: The first reason is the increase in the saturated vapour pressure of water (Option A). As temperature rises, the vapor pressure of water increases, making it easier for the bubble to expand as more water vapor can fill the bubble.
Step 3: The second reason is the root mean square velocity of air molecules inside the bubble increases (Option B). As temperature increases, the kinetic energy of the air molecules increases, leading to more frequent and forceful collisions between the air molecules, which causes the bubble to expand.
Step 4: The third reason is the decrease in surface tension of water (Option C). Heating water decreases its surface tension, allowing the bubble to grow larger more easily due to reduced cohesive forces acting on the surface of the bubble.
Conclusion: Since all three factors contribute to the increase in the size of the bubble upon heating the water, the correct answer is Option D: All of the above.