ANS:B - Cylinder

Cylinder is the right answer.

Tracks and Cylinders. The idea of a cylinder is an artifact of the mechanical nature of the actuators used to move the read/write heads on a modern disk. It is faster to read sectors from the same track than it is to move to another track, even one close by. We now consider the fact that standard disk drives with concentric tracks and multiple recording surfaces have the same number of tracks and track geometry on each of the surfaces.

When the actuator is functioning properly, as it almost always is, each read/write head is over (or near to) the same numbered track on its disk. This leads to the idea of a cylinder as the set of tracks that can be read without significant repositioning of the read/write heads. In early disk designs, with wider tracks, there would be no positioning required to move from surface to surface on a cylinder. All that would be required was an electronic switching of the active head, a matter of nanoseconds and not milliseconds.