Which of the following contains the instructions our CPU needs to understand external devices?

The correct choice is the one that refers to drivers. Drivers are specialized software that act as intermediaries between the operating system and external devices. They contain the necessary instructions and protocols that the central processing unit (CPU) needs to communicate with and control hardware components, such as printers, graphics cards, and network adapters.

When a new device is connected to a computer, the operating system uses the appropriate driver to interpret the signals sent by the device and to manage the operations that the device can perform. Without these drivers, the CPU would not know how to interact with these external devices, leading to failure in their functionality.

On the other hand, while the operating system does manage hardware resources and may have some built-in drivers, it is not the only component directly responsible for the specific instructions needed for different hardware devices. Software applications are created to perform specific tasks for users but do not directly interface with hardware in the way that drivers do. Firmware is also critical, as it provides low-level control for a device's specific hardware, but it is typically embedded within the device itself and not designed for general communication with the CPU.

Thus, drivers are essential for facilitating the correct and efficient operation of hardware by providing the necessary instructions the CPU requires to manage external devices.