• Cobalamin (Vitamin $B_{12}$):
• Vitamin $B_{12}$ is a water-soluble corrinoid ring complex that contains a trivalent cobalt atom ($Co^{3+}$) coordinated at the center of the corrin macrocycle.
• It acts as an essential cofactor for methionine synthase (homocysteine remethylation) and methylmalonyl-CoA mutase (propionate metabolism).
• Deficiency causes macrocytic normochromic anemia, poor growth, and mortality in fish.
• nalysis:
• (Vitamin B12): Correct.
• (Vitamin C): L-ascorbic acid (contains no metal ion).
• Options 3 & 4 (Vitamin A, Vitamin D): Lipophilic isoprenoid vitamins.

Correct option: (2) Vitamin B 42

• Proximate Composition Dynamics in Fish Flesh:
• In fish muscle tissue, a well-established classical inverse physiological relationship exists between water (moisture) and crude lipid (fat) content:

• As a fish accumulates energy stores (fat deposition during active feeding periods or high dietary lipid intake), water molecules within adipose and intracellular spaces are displaced by triglycerides.
• Conversely, during starvation or reproductive mobilization, lipids are depleted and replaced by water.
• nalysis:
• (Water): Correct. Water content is inversely proportional to fat content.
• Protein and Ash: Remain relatively stable across fluctuating nutritional states.

Correct option: (4) Water

• Fatty Acid Structure:
• Fatty acids are carboxylic acids comprising a terminal polar carboxyl group ($-COOH$) attached to an unbranched, non-polar aliphatic hydrocarbon chain represented as $R$:

• The radical $R$ represents an alkyl group (saturated hydrocarbon chain, $-C_nH_{2n+1}$) or an alkenyl group (unsaturated hydrocarbon chain containing one or more double bonds).
• nalysis:
• (Alkyl group): Correct.
• (Alkaloid group): Basic nitrogenous secondary plant metabolites.
• (Aldehyde group): Functional group with structure $-CHO$.
• (Acetyl group): Specific two-carbon acyl radical ($-COCH_3$).

Correct option: (1) Alkyl group