Amongst the given options which of the following molecules/ion acts as a Lewis acid?

A Lewis acid is a species that can accept a pair of electrons.${\text{NH}}_3,{\text{H}}_2\text{O},{\text{OH}}^{-}$ all have lone pairs available for donation, so they are Lewis bases.${\text{BF}}_3$: Boron has only 6 electrons in its valence shell. It is electron-deficient and can accept an electron pair from a donor to complete its octet. Therefore, it is a Lewis acid.