Answer: B. Acidified potassium manganate(VII) is a powerful oxidising agent: the manganese is reduced from +7 in the purple MnO₄⁻ ion to +2 in the colourless Mn²⁺ ion. The colour change from purple to colourless shows the manganate(VII) has been reduced.
If a substance reduces the manganate(VII), that substance must itself have donated electrons — in other words, X is a reducing agent.
Option A has the direction of electron transfer backwards. Options C and D are too restrictive: many substances decolourise acidified KMnO₄ (for example SO₂, Fe²⁺ and iodide ions), not just ethanol or sulfur dioxide.
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