What feature allows for the connection of an external DC power supply if the onboard batteries fail?

The feature that allows for the connection of an external DC power supply if the onboard batteries fail is the NATO and Aircraft plugs. These plugs are designed specifically for military and aviation applications, facilitating a reliable connection to external power sources. This capability is crucial in ensuring that equipment can maintain functionality even in the event of battery failure, which is particularly important in operational contexts where reliability is essential.

The NATO plug, for example, is standardized for military equipment and is compatible with various systems, allowing for quick and efficient power transfer. Aircraft plugs serve a similar purpose, enabling the integration of external power supply systems, which can also be found in various aircraft. By using these standardized connectors, the equipment can easily be serviced and powered up without needing to rely solely on the onboard energy source.

Other options do not assume the same level of standardized external compatibility required for such operations, making NATO and Aircraft plugs the vital choice in this scenario.