(a) $\mathrm{OH}^{-}$ : $\mathrm{OH}^{-}$ is a Lewis base because it can donate lone pair of electrons. (b) $\mathrm{F}^{-}$ : $\mathrm{F}^{-}$ is a Lewis base because it can donate lone pair of electrons. (c) $\mathrm{H}^{+}$ : $\mathrm{H}^{+}$ is a Lewis acid because it can accept lone pair of electrons. (d) $\mathrm{BCl}_3$ : $\mathrm{BCl}_3$ is a Lewis acid because it is electron deficient and can accept a lone pair of electrons.