$s=t^3-4t^2+7$
$\large\frac{ds}{dt}$$=3t^2-8t$
=> $\large\frac{d^2s}{dt^2}$$=6t-8$
Given acceleration =0
$6t-8=0$
$t =\large\frac{8}{6} =\frac{4}{3}$
Velocity $\large\frac{ds}{dt} $$=3 \times \large\frac{16}{9} $$ -8 \times \large\frac{4}{3}$
$\qquad= \large\frac{16}{3} -\frac{32}{3} =\frac{-16}{3} $$ m/sec$
Hence 2 is the correct answer