How do you know that intercostal drainage tube is functioning?

Correct option is C

Oscillation or tidaling in the water-seal chamber during respiration shows that the intercostal (chest) tube is patent and effectively responding to intrathoracic pressure changes. It confirms active communication between the pleural space and the drainage system.
Explanation of each option: • (a) Continuous bubbling in waterseal bottle – This is not normal and typically suggests an air leak in the system, either from the patient (e.g., pneumothorax) or from the tube connections. • (b) Continuous bubbling from suction tube – Bubbling in the suction control chamber is expected if wall suction is applied, but it doesn’t confirm whether the chest tube is properly draining or functioning. • (c) Oscillation of water column in drainage bottle – Correct. This tidaling movement, which rises during inspiration and falls during expiration, indicates a properly working and patent chest tube. • (d) No bubbling in the drainage bottle – Absence of bubbling does not indicate function; in fact, it might suggest tube blockage or lung re-expansion, depending on context. It must be interpreted with other signs.