Which of the following is an electron - deficient compound ?

The correct option is C $BC{l}_3$

In electron deficient compounds, the central atom either does not have eight electrons in the valence shell or it has eight electrons but can expand its valence beyond 4 due to presence of vacant d orbitals.

$BC{l}_3$ has 6 valence electrons on B atom and accepts pair of electrons from ammonia to complete its octet. Hence, it is an electron deficient compound.

Hence, the correct option is $\text{C}$