Which of the following is not a Lewis acid ?

The correct option is D $N{H}_3$
$SiC{l}_4$, $S{O}_3$ and $Z{n}^{2+}$, act as Lewis acids because of the ability of their central atoms to accept lone pairs in their empty orbitals.
But $CO$ does not have a vacant orbital. In fact due to the presence of lone pairs on carbon , it behaves like a lewis base.

Theory :
Lewis Concept (Electronic concept) :
Lewis acid - Species which can accept an electron pair with the formation of a coordinate bond.
Lewis base - Species which has a lone pair of electrons available for donation.