Phosphine gas will be produced if phosphorous acid (H₃PO₃) is subjected to:

Phosphorous acid (H₃PO₃) is a diprotic acid and an important compound of phosphorus that exhibits both acidic and reducing properties. Its molecular structure can be represented as HPO(OH)₂, where one hydrogen is directly bonded to phosphorus, and the other two hydrogens are part of hydroxyl groups.

When phosphorous acid is heated, it undergoes thermal decomposition, a chemical reaction in which a single compound breaks down into two or more simpler substances due to heat. The decomposition reaction proceeds as follows:

4H3PO3→3H3PO4+PH3↑4H₃PO₃ → 3H₃PO₄ + PH₃↑4H3PO3→3H3PO4+PH3↑

In this reaction:

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  Phosphine gas (PH₃), a colorless, flammable, and highly toxic gas with a fish-like odor, is liberated.

  
  


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  Phosphoric acid (H₃PO₄) remains as the other product.

  
  

This is not a simple oxidation or reduction reaction because no external oxidizing or reducing agent is used. However, the phosphorus atoms within the same molecule undergo disproportionation, where one atom is oxidized (forming H₃PO₄) and another is reduced (forming PH₃). Despite this internal redox change, the overall reaction type is categorized as thermal decomposition.

Phosphine gas formed in this process is used in semiconductor manufacturing, insecticidal fumigation, and sometimes in chemical synthesis, though it is handled with extreme caution due to its toxicity and spontaneous flammability in air.

In summary, the production of phosphine (PH₃) from phosphorous acid (H₃PO₃) occurs via decomposition, not through direct oxidation or reduction.

Hence, the correct answer is C) Decomposition.