Which of the following is NOT a type of hardware a cave diver may use?

The choice identifying flashlight holders as not being a type of hardware typically used by a cave diver is considered correct because flashlight holders, while they may be used to secure a light source, do not represent standard diving hardware in the same way that the other options do.

D-rings, for instance, are integral parts of a diver's harness system, providing points for attachment of various equipment. Shock cords are often used to secure items or keep them stowed properly, which is essential for preventing entanglement in a cave environment. A bicycle inner tube can be creatively repurposed by divers for various uses, such as flotation or securing equipment.

In summary, while all the items listed can play a functional role within the context of diving gear, only flashlight holders fall outside the category of essential, standardized diving hardware, making it the correct answer in this context.