According to the Lewis concept, a Lewis base is a species that donates an electron pair, while a Lewis acid is an electron pair acceptor.

$F^-$ has a negative charge and lone pairs of electrons, making it a good electron pair donor (Lewis base). $PF_3$ has a lone pair of electrons on the phosphorus atom, allowing it to act as a Lewis base. CO has lone pairs of electrons on both the carbon and oxygen atoms, enabling it to donate an electron pair and act as a Lewis base. $BF_3$, however, is an electron-deficient molecule. The central boron atom has only six electrons in its valence shell, meaning it needs an electron pair to complete its octet. Therefore, it acts as an electron pair acceptor (Lewis acid) and is least likely to act as a Lewis base.