Solution
Lewis acids are those compounds which have incomplete octate or complete octate which is able to keep more electrons in its outer shell.
SiCl
4
and SnCl
4
act as Lewis acids as they can extend their coordination number beyond 4 due to the presence of d−orbitals.
So they are able to keep greater than 8 electrons in its outermost orbit. Thus, increase its coordination number and they work as a Lewis acid.
CCl
4
can not keep more than 8 electrons in its outermost orbit due to the absence of d-orbitals. So, it not act as lewis acid.