Concept:Reducing a highly oxidized functional group like a carboxylic acid all the way down to a fully saturated alkane requires a tremendously aggressive reducing environment.
Formula:$$ \text{R-COOH} + 6\text{HI} \xrightarrow{\text{Red Phosphorus, } \Delta} \text{R-CH}_3 + 2\text{H}_2\text{O} + 3\text{I}_2 $$
Solution:- Hydrogen iodide (\( \text{HI} \)) in the presence of red phosphorus (\( \text{P} \)) is a powerful classical reagent system capable of completely stripping all oxygen atoms from a carboxylic acid, replacing them with hydrogens to form an alkane.
Why other options are incorrect:\( \text{LiAlH}_4 \) stops at the primary alcohol stage. \( \text{NaBH}_4 \) is too weak to reduce carboxylic acids. Catalytic hydrogenation (\( \text{Ni}/\text{H}_2 \)) is generally ineffective against the stable carboxyl group.
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