To solve this problem, we need to identify which of the given molecules or ions acts as a Lewis acid.$\newline$A Lewis acid is a chemical species that can accept an electron pair.$\newline$Let's analyze each option:$\newline$• Option 1: $NH_3$ is a Lewis base because it has a lone pair of electrons that it can donate.$\newline$• Option 2: $H_2O$ can act as a Lewis base due to the presence of lone pairs on the oxygen atom.$\newline$• Option 3: $BF_3$ is a Lewis acid because it has an incomplete octet and can accept an electron pair.$\newline$• Option 4: $OH^-$ is a Lewis base because it has a lone pair of electrons that it can donate.$\newline$Therefore, the molecule that acts as a Lewis acid is $BF_3. \newline$This corresponds to Option 3.