• Occurrence of Vitamin B12 in Nature:
• Vitamin B12 (Cobalamin) is unique among all vitamins because it is synthesized exclusively by anaerobic bacteria and archaea.
• Higher plants do not synthesize and do not require Vitamin $B_{12}$, meaning it is completely absent from unfortified food of plant origin (grains, legumes, fruits, vegetables).
• Animals obtain Vitamin $B_{12}$ by consuming animal products (liver, meat, milk, eggs) or through symbiotic gastrointestinal/ruminal microbial synthesis.
• nalysis:
• (Vitamin B12): Correct vitamin absent in plant sources.

Correct option: (3) Vitamin B12

• Size-Exclusion Chromatography Components:
• Stationary Phase: Porous, cross-linked, hydrated microscopic gel beads (e.g., dextran / Sephadex, agarose / Sepharose, polyacrylamide / Bio-Gel).
• Mobile Phase: A continuous flow of aqueous liquid buffer (liquid phase) carrying the dissolved protein mixture through the packed column bed under gravity or pressure.
• nalysis:
• (Liquid): Correct state of the mobile phase.

Correct option: (2) Liquid

• Mechanism of Gel Filtration:
• Assertion (A): Gel filtration chromatography separates macromolecules according to their hydrodynamic volume / molecular weight. (True).
• Reason (R): The column matrix consists of porous gel beads with defined pore dimensions. Large molecules exceeding the pore exclusion limit cannot enter the beads and travel solely through the interstitial void volume, eluting first. Smaller molecules enter the bead pores, traversing a tortuous path with increased retention volume, and elute at a later stage. (True).
• nalysis:
• Both (A) and (R) are correct, and (R) is the precise physical explanation for (A). Hence, Option 1 is correct.