The correct option is B C2H6
Number of electrons in [H3N:BH3]=10+8=18
(a) B2H6:
Number of electrons : 10+6=16
(b) C2H6(12+6=18) has 18 electrons and [NH3+BH3]=10+8=18. So, both are isoelectronic with each other.
(c) C2H4
Number of electrons : 12+4=16
(d) C3H6
Number of electrons : 18+6=24.