Which of the following compound CANNOT act as a Lewis base?

A Lewis base is a species that can donate a lone pair of electrons. For a molecule to act as a Lewis base, its central atom must have at least one lone pair of electrons available for donation.

In $$NF_3$$, nitrogen has one lone pair and can donate it, so it acts as a Lewis base. In $$SF_4$$, sulfur has one lone pair available for donation. In $$ClF_3$$, chlorine has two lone pairs and can donate one of them.

However, in $$PCl_5$$, phosphorus uses all five of its valence electrons to form five bonds with chlorine atoms. The phosphorus atom has no lone pairs remaining, meaning it cannot donate any electron pair to act as a Lewis base. In fact, $$PCl_5$$ is an electron-deficient species at phosphorus and tends to act as a Lewis acid instead.

Therefore, the compound that cannot act as a Lewis base is $$PCl_5$$, which is option (B).