For this type of questions I follow a simple trick , no need of options in this and surely you get verified answer.
Step 1: find the cube root of given total number of routers.
Cube root(5760) = 17.96
On dividing 5700 by 17 or 18 we couldn't find a perfect number whereas on going with 16 we will find a solution . (You can go on increasing the number but rather I suggest to check for number in close proximity of cube root)
5760/16 = 360
Step 2: find the square root this number
Sqrt(360) = 18.97
Again on doing division by 18 only we will find
360/18 = 20
Now we have three numbers {16, 18, 20}
We know the relationship as
Number (clusture) < Number(regions) < Number(routers)
So we have
# of clustures = 16
# of regions = 18
# of routers = 20
Size of the table = 16+18+20
=54
@Arjun sir please varify my answer if there is any mistake.