Statement (d) is correct, while other statements are incorrect. Corrected form are as follows :
(a) The four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms.
(b) The four terminal B−H bonds are regular 2 -centre-2-electron bonds.
(c) The two bridged (B−H−B) bonds are different and can be described in terms of 3 -centre-2-electron bonds.