• Fischer Projection and Stereochemical Configuration:
• Fischer projections represent stereocenters with vertical bonds extending away from the viewer (into the page) and horizontal bonds projecting toward the viewer (out of the page).
• Rotation of a Fischer projection by $180^\circ$ in the plane of paper maintains absolute $(R/S)$ configuration, whereas an odd number of pairwise group interchanges inverts configuration to an enantiomer.
• Projections 1, 2, and 4 represent equivalent or diastereomerically consistent conformations of the molecule, whereas projection 3 possesses an inverted stereochemical configuration at the chiral center.
• Conclusion: Projection 3 is stereochemically different from the other three projections (Option 3).

Correct option: (3) 3

• Homopolymer vs Copolymer Classification:
• homopolymer is formed through polymerization of a single unique monomeric repeating unit.
• Neoprene (Polychloroprene) is a homopolymer synthesized solely by free-radical polymerization of chloroprene (2-chloro-1,3-butadiene):

$$n\ [\text{CH}_2=\text{C}(\text{Cl})-\text{CH}=\text{CH}_2] \longrightarrow -[-\text{CH}_2-\text{C}(\text{Cl})=\text{CH}-\text{CH}_2-]_n-$$

• Copolymers: Bakelite (phenol + formaldehyde), Nylon-66 (adipic acid + hexamethylenediamine), and Terylene/Dacron (terephthalic acid + ethylene glycol) are synthesized from two distinct monomeric precursors.
• Conclusion: Neoprene is a homopolymer (Option 4).

Correct option: (4) Neoprene

• Biopolymer Monomers and Structure:
• Chitin is an abundant structural polysaccharide composed of linear $\beta\text{-(1}\rightarrow\text{4)}$-linked units of $N$-acetyl-D-glucosamine (and deacetylated D-glucosamine).
• It forms the structural exoskeleton of arthropods (insects, crustaceans) and the fungal cell wall.
• Other options: Pectin consists primarily of $\alpha\text{-(1}\rightarrow\text{4)}$-D-galacturonic acid; Alginic acid consists of $\beta$-D-mannuronate and $\alpha$-L-guluronate; Inulin consists of fructofuranosyl units.
• Conclusion: D-glucosamine is the building block of Chitin (Option 4).

Correct option: (4) Chitin