Which device consists of a free turning grooved metal wheel to reduce rope friction?

The device that consists of a free turning grooved metal wheel to reduce rope friction is a pulley. A pulley allows for the rerouting of rope, helping to lift loads more efficiently by reducing friction at the point where the rope passes over the wheel. This design contributes significantly to improving the mechanical advantage during lifting or lowering operations, which is essential in rescue scenarios.

While the other options serve specific functions in rope rescue, they do not fit the characteristics described in the question. A descender is primarily used for controlled descent and is typically designed to manage braking and direction of rope, but it does not reduce rope friction in the same way as a pulley does. Similarly, a figure 8 descender is a specific type of descender that provides a controlled descent but is also not intended primarily for friction reduction. A rope grab is a device designed to grip the rope and automatically lock in the event of a fall; its function is more about safety and security in movement rather than reduction of friction.