Correct Answer - B
`f_("limiting")=mumg=0.3xx2.10=6N`
Force needed for accn of `2m//s^(2)`
`F=ma=4N`
So friction force is static `f=4N`
displacement w.r.t observer in `t=2` sec is
`S=U_("rel")t+1/2a_("rel") t^(2)=01/2(-3)xx4=-6m`
`W=vecf.vecS=4(-6)=-24J`