B2H6
SiH4
C2H6
PH3
B2H6 is an electron-deficient molecule because the boron atom has three half-filled orbitals in an excited state. The structure of B2H6 is represented as follows:
In it, two electrons of a B-H bond are involved in the formation of a three-center two-electron bond; these bonds are represented by a dotted line. Hence, option A is correct.