Which from the following is Lewis base?

A Lewis base is a chemical species that can donate a pair of non-bonding electrons.

$\text{AlCl}_3$ and $\text{BF}_3$ are electron-deficient molecules, as the central atoms (Al and B) have incomplete octets. They accept electron pairs and act as Lewis acids.

$\text{H}^+$ is a proton with no electrons, so it accepts an electron pair and acts as a Lewis acid.

In $\text{H}_2\text{O}$, the oxygen atom has two lone pairs of electrons that it can donate. Therefore, $\text{H}_2\text{O}$ acts as a Lewis base.

Answer: $\text{H}_2\text{O}$