Answer A)49
With no compartment engine speed 42kmph
24 kmph speed when 9 compartment attached
reduction in speed is directly proportional to the square root of the number of compartments attached
So, when 9 compartment attached speed reduction 18kmph
18 ∞ √9
18=3k
or, k=6
So, for 42kmph
42=6√x [where x number of compartment]
or, x=49