Let assume we have $x$ number of platters.
Capacity of the hard disk =$30GB$=$30*2^{30} B$
Now the disk has $4096$ tracks/platters.
we have x number of platters so no of tracks we have =$x*4096$
It has $2048$ sector/track.
we have $x*4096$ number of tracks so number of sector we have =$x*4096*2048$
Each sector has capacity=1024B
so total capacity of the disk =$x*4096*2048*1024 $ Byte
so it is same as $30*2^{30} B$.
By equating them,
$x*4096*2048*1024=30*2^{30}$
$x=\frac{30}{8}=3.75$
so we need 4 platters for this Disk.
Correct answer is option 3.
If someone has doubt why we need 4 platters why not 3 platters if we have used 3 platters then the capacity of the disk =$3*1024*2048*4096$=24GB but in the question we have given 30 GB as the disk capacity so using 3 platters we cannot get 30GB capacity.