Correct Answer - A
As water is considered to be incompressibel , so volume covered by inward movement of smaller piston = volume covered outward movement of larger piston.
`L_(1)A_(1) = L_(2)A_(2)`
`L_(2) = (A_1)/(A_2)L_(1) = (pi(D_(2)//2)^(2))/(pi(D_(1)//2)^(2)) L_(1) = ((D_1)/(D_2))^(2)L_(1)`
`=((1cm)/(3cm))^(2)xx6 cm = 6/9 cm = 2/3cm`.