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.