Solution

The correct answer is 169 cylinders 

EXPLANATION:

In the SCAN algorithm (also known as the elevator algorithm), the disk arm starts from one end of the disk and moves towards the other end, servicing requests until it gets to the other end. After reaching the other end, it reverses its direction and again services the requests coming in its path.

In this case, the head is currently at track 143 and moving towards higher-numbered tracks (because it just serviced a request at track 125 and is now at track 143).

Given the queue of requests: 86, 147, 91, 177, 94, 150, 102, 175, 130.

We first sort the queue in ascending order but consider only those, which are in the direction of the head movement: 147, 150, 175, 177.

Then, it reverses its direction and services the remaining requests.

Total number of head movements = (199 -143) + (199 - 85) 

Total number of head movements = 56 + 113 = 169