Fajan's rules describe the conditions under which an ionic bond develops partial covalent character. The covalent character of an ionic bond is favored by:

• High polarizing power of the cation: This is favored by a small size and high charge of the cation. A smaller cation can more effectively distort the electron cloud of the anion.
• High polarizability of the anion: This is favored by a large size and high charge of the anion. A larger anion has a more diffuse electron cloud that can be more easily distorted by the cation.

Therefore, a small size cation and a large size anion will maximize the polarizing power of the cation and the polarizability of the anion, respectively, leading to increased covalent character.

• A) large size cation and small size anion: A large cation has low polarizing power, and a small anion has low polarizability. This combination favors ionic character.
• B) small size cation and large size anion: A small cation has high polarizing power, and a large anion has high polarizability. This combination favors covalent character, which is consistent with Fajan's rules.
• C) large size cation and large size anion: While a large anion has high polarizability, a large cation has low polarizing power. The overall effect would be less covalent character compared to option B.
• D) small size cation and small size anion: While a small cation has high polarizing power, a small anion has low polarizability. The overall effect would be less covalent character compared to option B.

Correct Answer: (B)