Which of the following is the major part of time taken when accessing data on…

Concept: The total time to access data on a disk is made up of several distinct delays that happen before the transfer itself: seek time (the read/write head physically moving to the correct track), rotational latency (waiting for the target sector to rotate under the head), and settle time (a brief pause for head vibration to damp out once it reaches the track). Because seek time involves mechanically repositioning the actuator arm — a comparatively slow, physical motion covering anywhere from one to hundreds of cylinders — it is typically the largest of these components, often several times longer than rotational latency and orders of magnitude longer than settle time.

Application: For this question, the head must first travel to the target track before any reading or writing can start, and that mechanical travel consistently takes longer (commonly several milliseconds) than the roughly half-a-revolution wait for rotational latency or the sub-millisecond settle time. This is exactly why disk-access-time analysis (and standard OS/COA references) identifies this head-positioning delay as the dominant, major part of total disk access time.

Cross-check — why the other listed timings are not the major part:

• Settle time is only the microsecond-scale pause for head vibration to damp once positioning is done — a small fraction of the total, not the major part.

• Rotational latency is bounded purely by the disk's spin speed (on average, half a revolution) — a real delay, but one that doesn't involve any mechanical repositioning, so it is generally smaller.

• Waiting time (queueing time) is the delay a request spends waiting in a scheduling queue before the disk even starts servicing it — that is a contention/scheduling concept external to the disk's own physical access-time components, so it cannot itself be counted as part of the disk's access time.