Question

According to the Lewis acid-base concept:
(A) Species in which the central atom has completed octet cannot act as an acid
(B) Negatively charged ions are acids
(C) Molecules in which a central atom has vacant d-orbitals available acts as an acid
(D) All positively charged ions are based

Answer 1

According to Lewis acid-base concept: If a substance is able to accept a pair of the electron because it has vacant orbitals then it will act as an acid and if the substance is able to donate a pair of the electron due to the presence of lone pair then it is a base.

Complete step by step solution:
Many theories came one by one for defining the nature of an acid and a base. Bronsted-Lowery's concept of acids and bases was broader than the Arrhenius concept in the sense that it could not explain not only all the Arrhenius acids and bases but included many more substances as bases. However, it did not have a much larger scope for acids.
So, G.N. Lewis in 1923, proposed a broader concept of acids and bases. Therefore, this theory explained that:
If a substance is able to accept a pair of the electron because it has vacant orbitals then it will act as an acid and if the substance is able to donate a pair of the electron due to the presence of lone pair then it is a base.
Some examples of acids are $B{{F}_{3}},\text{ }AlC{{l}_{3}},\text{ }FeC{{l}_{3}}$ etc and some examples of bases are $N{{H}_{3}},\text{ }RN{{H}_{2}},\text{ }ROH$ etc.

So, the correct answer is an option (C)- Molecules in which a central atom has vacant d-orbitals available acts as an acid.

Note: It is obvious that when the reaction takes place between an acid and a base according to this concept, then the bond will be a coordinate bond or a dative bond between them.