Final answer:
The Data out register is used by the host system to send output to a peripheral device. It acts as a buffer in the interface between the system's bus and the external device.
Step-by-step explanation:
The register that is written by the host to send output is the d) Data out register. When a system wants to output data to a device, such as writing data to a disk or sending information to a printer, it will use the Data Out register. This register acts as a buffer for the data that the system wants to send to an external device; it is part of the interface between the system's bus and the peripheral device.