According to the Lewis concept, a base is a chemical species that can donate an electron pair, while an acid is a species that can accept an electron pair.

• $NH_3$: The central nitrogen atom has one lone pair of electrons, so it can act as a Lewis base.
• $OH^-$: The oxygen atom has lone pairs and a negative charge, making it a strong Lewis base.
• $H_2O$: The oxygen atom has two lone pairs of electrons, allowing it to act as a Lewis base.
• $BF_3$: The central boron atom has only 6 electrons in its valence shell (an incomplete octet). Being an electron-deficient species, it acts as an electron pair acceptor, i.e., a Lewis acid. Therefore, it is least likely to behave as a Lewis base.