The maximum number of atoms present in the same plane in diborane molecule is

I) In diborane each B atom undergoes sp3  hybridisation ,it contains two B-H-B bridges . Each bridge bond is formed by overlapping of one "B"sp3  hybrid orbital with one electron, hydrogen s-orbital with one electron and another boron sp3 hybrid orbital without electron .That is the bridge head bond is a electron deficient three centre two electron bond .The number of atoms involved in bridged bonds in one diborane molecule is four.                                                                                                                                                   Total six atoms are present in same plane.In diborane two BH2 groups are in one plane and two bridged hydrogens are perpendicular to the BH2 plane.Therefore it is a non planar molecule and the central atom boron undergoes sp3 hybridisation.