A burst disc on a tank valve is activated by which of the following?

The burst disc on a tank valve is designed to provide a safety mechanism by preventing over-pressurization of the tank. When the pressure inside the tank exceeds a predetermined threshold, the burst disc will rupture, thereby releasing gas or liquid from the tank in a controlled manner. This helps to prevent catastrophic failure of the tank which could result in a more dangerous situation.

The other choices represent different concepts that do not activate the burst disc directly. While extreme heat can lead to increased pressure, it is not the direct activation mechanism for the disc itself; rather, the disc reacts specifically to pressure levels. A J-valve reserve device is unrelated to pressure regulation and is instead a mechanism that allows divers to access a reserve of air. Similarly, a cylinder explosion would not activate a burst disc; rather, it indicates a failure scenario rather than a preventive measure. Thus, the focus on over-pressurization captures the essential function of the burst disc effectively.