In the ________ disk scheduling algorithm, the disk head moves from one end of the disk to the other, servicing requests along the way. When it reaches the other end, it immediately returns to the beginning of the disk without servicing any requests on the return trip.

Explanation

The described behaviour — sweep from one end to the other servicing requests, then return to the start without servicing anything on the way back — is the defining feature of C-SCAN (Circular SCAN), which treats the cylinders as a circular list so every region of the disk gets a more uniform wait time. Plain SCAN (the 'elevator' algorithm) differs because it keeps servicing requests on the return sweep too, reversing direction only at the disk's physical ends. LOOK and C-LOOK are variants that reverse at the last pending request in each direction rather than going all the way to the disk's end, so they don't match the wording either. Only C-SCAN matches every detail given, so the correct answer is C-SCAN.