In the formation of B–H–B bridge in diborane, the orbitals involved in overlapping are

1s orbital of ‘H’ atom and one of the each ${\mathrm{sp}}^3$ hybrid orbital of Boron atoms containing one electron each 

1s orbital of ‘H’ atom and one of the each vacant  hybrid orbital of Boron atoms

1s orbital of ‘H’ atom and vacant  orbital of one Boron and  orbital of another boron containing one electron 

1s orbital of ‘H’ atom and vacant  orbital and orbital containing single electron of the same Boron atom