In potential energy curve, the point at maximum (i.e. at crest) is the point of unstable equilibrium since, the displacement of particle towards left or right will experience the increase or decrease in the force and potential energy, both are related by the relation,

$\mathrm{F}=-\frac{\mathrm{\delta U}}{\mathrm{\delta x}}$

Hence, The point at unstable equilibrium is point $\mathrm{B}$