Correct Answer - B
The reagent is `(BH_(3))_(2)` called diborane. It it is the dimer of the hypothetical `BH_(3)` (borane) but in the hydroboration reaction, it acts much as though it were `BH_(3)`. In fact, when diborane dissolves in tetrahydrofuran (one of the solvent used for hydroboration), `THF`, each `B_(2)H_(6)` molecule dissociates to produce two molecules of a complex between `BH_(3)` (called borane) and `THF`, i.e., the reagent exists as the monomer in the from of an acid-base complex with the solvent.
`BH_(3)` is Lewis acid beacuse the central `B` atom has only six electrons in its valence shell. It accepts `e^(-)` pair from the `O` atom of `THF`. Solutions containing the `THF:BH_(3)` complex can be obtained commercially.
