Q.Which of these is not likely to act as a Lewis base?

Gilbert N. Lewis's 1923 concept is the most general of the three, defined by the electron pair rather than the proton: a Lewis acid is a species that accepts an electron pair, and a Lewis base is a species that donates an electron pair. A Lewis acid is typically a positive ion or an electron-deficient molecule; a Lewis base is typically an anion or a neutral molecule with at least one lone pair.

In $BF_3+NH_3 \rightarrow F_3B{\leftarrow}NH_3$, boron's vacant $2p$ orbital accepts the lone pair donated by nitrogen, forming a new coordinate covalent bond: $BF_3$ is the Lewis acid and $NH_3$ is the Lewis base. In $Cr^{3+}+6H_2O \rightarrow [Cr(H_2O)_6]^{3+}$, each water molecule donates a lone pair to $Cr^{3+}$, so $Cr^{3+}$ is the Lewis acid and $H_2O$ is the Lewis base -- the same logic underlies every coordination compound, where the ligands are Lewis bases and the central metal ion is the Lewis acid.

Families of Lewis acids: electron-deficient molecules ($BF_3, AlCl_3, BeF_2$), all metal ions ($Fe^{2+}, Cr^{3+}, Cu^{2+}$), molecules with a polar double bond ($SO_2, CO_2, SO_3$), molecules whose central atom can expand its octet via empty d orbitals ($SiF_4, SF_4, FeCl_3$), and carbonium ions ($CH_3^+$).

Families of Lewis bases: molecules with one or more lone pairs ($NH_3, H_2O, R{-}O{-}H$), all anions ($F^-, CN^-, SO_4^{2-}$), molecules with a carbon-carbon multiple bond ($CH_2{=}CH_2, CH{\equiv}CH$), all metal oxides ($CaO, MgO, Na_2O$), and carbanions ($CH_3^-$). …