• Vitamin D Potency Equivalence:
• By international definition, $1\;\mu\text{g}$ of crystalline cholecalciferol (Vitamin $D_3$) provides $40\text{ IU}$ of biological activity.
• Therefore, for $1\text{ mg}$ ($1000\;\mu\text{g}$):

(In the question options, $40\text{ IU}$ per $\mu\text{g}$ or $40,000\text{ IU}$ per mg is denoted).

Correct option: (4) 40 1U

• Star-Gazing Posture (Opisthotonos) in Chicks:
• Characterized by extreme retraction of the head back over the spine with loss of equilibrium.
• Pathognomonic clinical sign of Thiamine (Vitamin $B_1$) deficiency.
• Biochemical Basis: Thiamine pyrophosphate (TPP) deficiency impairs pyruvate dehydrogenase activity, leading to accumulation of pyruvate and lactic acid in the central nervous system.

Correct option: (3) Thiamine

• Limit Dextrin Digestion:
• Alpha-amylase cannot hydrolyze $\alpha(1\to 6)$ glycosidic branch linkages in amylopectin, leaving highly branched "limit dextrins".
• Debranching enzymes (isomaltase / oligo-1,6-glucosidase) cleave these $\alpha(1\to 6)$ bonds, ultimately hydrolyzing the branched oligosaccharides into free glucose for intestinal absorption.

Correct option: (3) Glucose

• Laxative Effect in Poultry Diets:
• Copra Meal (Coconut Meal) has a high concentration of water-soluble galactomannans and non-starch polysaccharides (NSPs) that absorb water in the gut and produce loose, watery droppings (laxative effect) when incorporated at $\gt 10\text{--}15\%$ in poultry rations.

Correct option: (2) (C) only.