Discussion
When actually data is to be read, the first action taken by device driver is :
Answer: D
The key gives to request a buffer space to receive data as the device driver's first action when data is to be read.
In the usual account of how a blocking read is served, the kernel's I/O subsystem passes the request to the device driver. The driver first obtains a kernel buffer in which the incoming data will be placed and then schedules the I/O. Only after that does it write commands into the device controller's registers, wait for the device, and copy the data out when the interrupt arrives. Lecture notes from several operating-systems courses describe the driver reserving the buffer before the transfer is started.
Checking the device status and sending the read message or a get-ready signal to the controller come later in the sequence, once the destination for the data exists. I could not find a source that words the sequence exactly as this question does, and some texts start the driver's work by checking whether the device is busy, so this follows the official key and the buffer-first description.
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