• Gel Filtration Chromatography (Size-Exclusion Chromatography - SEC):
• Principle: Separates molecules based on differences in their hydrodynamic size and shape as they pass through a column packed with porous gel beads (e.g., Sephadex, Sepharose).
• Applications:
• Purification by shape and size: Larger molecules are excluded from bead pores and elute first in the void volume ($V_o$), while smaller molecules penetrate pores and elute later.
• Desalting: Rapid separation of large macromolecules (proteins) from small salt ions or free dye.
• Analyzing protein-protein interactions: Determination of native molecular mass, quaternary structure, and stable protein complex formation.
• Charge-based separation: Carried out by ion-exchange chromatography, not gel filtration.
• Conclusion: Gel filtration is used for (A), (B) and (C) only.

Correct option: (1) (A), (B) and (C) only.

• Cation Exchangers in Chromatography:
• Cation-exchange resins possess negatively charged functional groups that attract and bind positively charged molecules (cations):
• Carboxy-Cellulose and (D) Carboxymethyl-Cellulose (CM-cellulose): Weak cation exchangers bearing carboxylate groups ($-\text{O}-\text{CH}_2-\text{COO}^-$).
• Sulfopropyl (SP) / Methyl sulfonate (S): Strong cation exchangers ($-\text{SO}_3^-$).
• Anion Exchangers: Possess positively charged groups that bind anions:
• DEAE-Sepharose: Diethylaminoethyl, a weak anion exchanger ($-\text{CH}_2\text{CH}_2\text{N}^+\text{H}(\text{CH}_2\text{CH}_3)_2$).
• Q-Sepharose: Quaternary ammonium, a strong anion exchanger ($-\text{CH}_2\text{N}^+(\text{CH}_3)_3$).
• Conclusion: Resins (C) and (D) only are cation exchangers.

Correct option: (4) (C) and (D) only.