a) In a two center electron pair bond, a pair of electrons binds two atoms, whereas in a three center electron pair bond a pair of electrons binds three atom together.
b) LiAlH4 is a more versatile reducing agent than NaBH4; it reduces as many as sixty functional groups in organic chemistry. On the other hand, NaBH4 is more selective. For example, NaBH4 reduces >C=O group in aldehyde and ketones to alcohols, but functional groups such as >C=C<, —COOH and —NO2 are not normally attacked.
c) i) NaBH4 ii) LiAlH4