A scuba cylinder must be hydrostatically tested:

A scuba cylinder must undergo hydrostatic testing within fixed intervals set by local laws because this testing is mandated to ensure the integrity and safety of the cylinder used for storing high-pressure gases. This process checks for structural integrity and verifies that the cylinder can safely hold the pressure it is designed for. Regulations vary by region, which is why local laws dictate the specific intervals for testing, ensuring compliance with safety standards and preventing potential failures that could lead to accidents.

The other options suggest intervals or conditions that do not align with industry standards or safety practices. For example, testing every six months is impractical and unnecessary based on the durability of well-maintained cylinders. Visually inspecting a cylinder is important but is not a substitute for hydrostatic testing, which measures the actual resilience of the tank under pressure. Similarly, requiring testing every two years does not account for local regulations that may mandate different time frames based on the jurisdiction.