Correct Answer - C
Let K be the initial kinetic energy and F be the resistive force. Then according to work-energy theorem,
`W=DeltaK`
i.e., `3F=1/2mv^2-1/2m(v/2)^2` …(1)
For B to C:
`Fx=1/2m(v/2)^2-1/2m(0)^2` …(2)
Dividing eqns. (1) and (2) we get `x/3=1/3`
or `x=1cm`