The potential energy of a particle moving along $x-$ axis is given by $U=20+5\sin \left(4\mathrm{\pi x}\right)$, where $U$ is in $J$ and $x$ is in metre under the action of conservative force:

if total mechanical energy is $20 J$, then at $x=\raisebox{1ex}{$7$}\!\left/ \!\raisebox{-1ex}{$8$}\right.m$, particle is at equilibrium

if total mechanical energy is $20 J$, then at $x=\raisebox{1ex}{$7$}\!\left/ \!\raisebox{-1ex}{$8$}\right.m$ particle is not at equilibrium

if total mechanical energy is $20 J$, then at $x=\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$8$}\right.m$, particle is at equilibrium

if total mechanical energy is $20 J$, then at $x=\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$8$}\right.m$, particle is not at equilibrium