What is a key reason divers should ensure they have a functional tank before diving?

Ensuring that a dive tank is functional before diving is crucial primarily to avoid emergency situations underwater. A diver relies on the tank as a primary source of breathing gas while submerged. If the tank has any faults or is not filled, it could lead to an oxygen shortage, which may create a life-threatening situation. This is particularly critical at greater depths, where emergencies can escalate quickly due to depth and pressure-related factors.

Without a functional tank, a diver could find themselves in a distressing scenario where they cannot access breathable air, which could lead to panic, rapid ascent, or other complications like decompression sickness. Maintaining a reliable breathing supply is fundamental to safety and the overall success of the dive. The other options, while they relate to different aspects of diving, do not address the immediate and life-critical nature of having a functioning tank for underwater breathing.