What is the operating principle upon which a bubble tube system operates in indicating liquid tank level?

Correct Answer: Option C — The air pressure required to force air bubbles into the liquid near the bottom of the tank is proportional to the liquid level in the tank.

Bubble tube systems utilize hydrostatic pressure. By forcing air through a tube submerged to the tank bottom, the required air pressure balances the liquid head. Because pressure at the bottom is directly proportional to the liquid height (P=ρgh), the measured air pressure provides an accurate, continuous indication of tank level.