Capacity for 1 platter = Number of track/surface * Number of sector/track * sector size
$= 2000 * 50 * 512 = 51200000 $Bytes
Since it has two surface, it becomes $102400000$ Bytes
For 5 platters, i.e capacity of disk $= 5*102400000 = 512000000$ Bytes $= 500000$ KB
Option B) is correct