What primary capability must a belay device have in rope rescue situations?

In rope rescue situations, a belay device must have the ability to lock under load. This capability is crucial because, during a rescue operation, the safety of both the rescuer and the subject being rescued is paramount. Locking under load ensures that if there is a sudden drop or shift in weight, the device will automatically secure the rope and prevent any unintentional descent or slippage. This provides a reliable safety mechanism, allowing the rescuers to focus on their task without the fear of losing control of the rope system.

Other options, while they might have their place in specific scenarios or equipment, do not directly address this primary need for safety and security in a belay device. For instance, while the ability to release the line automatically could offer convenience, it poses a risk of accidental release in high-stakes situations. Cutting the rope easily is not typically a feature considered essential for belay devices and, in fact, could create dangerous situations if misused. Lastly, while using multiple ropes can be important for some operations, the primary function in a rescue scenario remains to securely hold and manage a single rope under load.