Which of the following is an electron deficient compound?

The correct option is C $BC{l}_3$
If an atom has less than eight electrons in its valence shell after forming a molecule, the molecule is called an electron-deficient compound.

Boron has three valence electrons. Three chlorine atoms share one electron each with boron to form $BC{l}_3$ molecule. After the formation of the molecule, boron only has six electrons in its valence shell.

Since in $BC{l}_3$ boron has less than eight electrons in its valence shell, it is an electron-deficient compound.