According to the definition a molecule which can accept a lone pair is called a lewis acid. A) Ammonia has a lone pair on nitrogen, so it can donate the lone pair rather than accepting a lone pair. So, it is a lewis base. B)Water has 2 lone pairs on oxygen so it cannot accept any further lone pairs, so water is a lewis base not a lewis acid. C)In diborane the bonds found are banana bonds or tau bonds so it has a tendency to accept a lone pair because it has empty orbitals.So it can be considered as a lewis acid. D)Carbon usually doesn't accept or donate lone pair, so it is neither a lewis base nor lewis acid.We can consider it as a neutral molecule.