• Sucrose biosynthesis: In the cytosol, glucose-1-phosphate reacts with UTP via UDP-glucose pyrophosphorylase to generate UDP-glucose, the glucosyl donor for sucrose-phosphate synthase -> (A) - (IV).
• Nitrite reduction: Nitrite reductase complexes utilize reduced flavin adenine dinucleotide (FADH2) / reduced ferredoxin electron shuttles -> (B) - (III).
• Fatty acid biosynthesis: De novo fatty acid synthesis in plastids requires massive amounts of reducing power in the form of NADPH (for $\beta$-ketoacyl-ACP reductase and enoyl-ACP reductase) -> (C) - (II).
• Substrate-level phosphorylation in TCA cycle: Catalyzed by succinyl-CoA synthetase, where the cleavage of the thioester bond directly couples with the phosphorylation of GDP to yield GTP -> (D) - (I).
• Thus, the correct match is (A) - (IV), (B) - (III), (C) - (II), (D) - (I) (Option 2).

Ionization of polyprotic phosphoric acid ($\text{H}_3\text{PO}_4$):

• Stepwise dissociation: $\text{H}_3\text{PO}_4 \rightleftharpoons \text{H}^+ + \text{H}_2\text{PO}_4^-$, $K_{a1} = 7.1 \times 10^{-3}$, $\text{p}K_{a1} \approx 2.15$.
• Second dissociation: $\text{H}_2\text{PO}_4^- \rightleftharpoons \text{H}^+ + \text{HPO}_4^{2-}$, $K_{a2} = 6.3 \times 10^{-8}$, $\text{p}K_{a2} \approx 7.20$.
• Third dissociation: $\text{HPO}_4^{2-} \rightleftharpoons \text{H}^+ + \text{PO}_4^{3-}$, $K_{a3} = 4.2 \times 10^{-13}$, $\text{p}K_{a3} \approx 12.38$.
• Because each successive proton is removed from an increasingly negative species, $K_{a1} \gt K_{a2} \gt K_{a3}$, meaning $\text{p}K_{a1} \lt \text{p}K_{a2} \lt \text{p}K_{a3}$.