• 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

• Evaluation of Assertion and Reason:
• Assertion (A): The LDH isoenzyme $M_4$ functions optimally in the anaerobic environment of skeletal muscles.
• True. The muscle-type isozyme ($M_4$ / LDH-5) has a high $K_m$ for pyruvate and is not inhibited by high substrate concentrations, operating efficiently during intense anaerobic exercise.
• Reason (R): Skeletal muscles are designed to work anaerobically so as $M_4$. $M_4$ converts pyruvate to lactate, so glycolysis can be operated continuously.
• True. During vigorous muscular contraction when $O_2$ is limiting, $M_4$ rapidly reduces pyruvate to lactate, re-oxidizing $NADH$ back to $NAD^+$. Continuous regeneration of cytosolic $NAD^+$ is obligatory for glyceraldehyde-3-phosphate dehydrogenase, allowing anaerobic glycolysis and ATP production to proceed uninterrupted.
• nalysis:
• Both (A) and (R) are correct, and (R) provides the definitive biochemical explanation for (A). Hence, Option 1 is correct.

• 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.