Boron atom in diborane is known to be sp3 hybridised because it has four hybrid-orbitals. Three of these has one electron each and one is an empty orbital. The two electrons of hybrid orbitals in each of boron atom form two bonds with 1s of hydrogen atoms. The two atoms of boron left with that of each unpaired electron orbital and empty orbital forms two bridging (B−H−B) bonds with that of two 1s hydrogen atoms. It is also called banana bond. Hence, option (c) is the correct answer.