• Branchial Compensation of Respiratory Acidosis:
• Environmental hypercapnia (elevated water $CO_2$) causes hydration of $CO_2$ in fish blood, producing carbonic acid and driving down intracellular and extracellular $\text{pH}$ (respiratory acidosis):

• Fishes compensate by actively excreting $H^+$ ions and retaining/accumulating bicarbonate ions ($HCO_3^-$) via branchial gill epithelia exchangers ($Na^+/H^+$ and $Cl^-/HCO_3^-$), elevating plasma bicarbonate to restore systemic blood $\text{pH}$.
• Correct Option: Option 4 (Bicarbonate concentration).

• Antifreeze Glycoproteins (AFGPs) in Polar Teleosts:
• Found in Antarctic notothenioids (e.g., Trematomus, Pagothenia) and Arctic cod, AFGPs prevent ice crystal growth in body fluids by non-colligative thermal hysteresis (depressing freezing point without altering melting point).
• Primary Structure:
• AFGPs consist of repetitive tandem tripeptide units:

• Each Threonine residue is covalently $O$-glycosidically linked to the disaccharide $\beta\text{-D-galactosyl-(1}\rightarrow 3)\text{-}\alpha\text{-D-N-acetylgalactosamine}$.
• nalysis:
• (Alanine-alanine-threonine): Correct repeating amino acid sequence.

Correct option: (3) Alanine-alanine-threonine

• Origin and Nomenclature of C-Reactive Protein:
• Discovered in 1930 by Tillett and Francis, CRP is a prototypical acute-phase pentraxin protein of the innate immune system present in vertebrates, including fishes.
• It was originally named for its ability to form a precipitate in the presence of $Ca^{2+}$ with the "Fraction C" polysaccharide (C-substance, a teichoic acid) extracted from the cell wall of Streptococcus pneumoniae.
• Subsequent biochemical research revealed that CRP specifically binds to the phosphocholine (phosphorylcholine / phosphatidylcholine) headgroup of this C-polysaccharide and exposed cell membrane phospholipids.
• nalysis:
• (Phosphatidylcholine / Phosphocholine): Correct.

Correct option: (4) Phosphatidylcholine