Answer: A (all three methods, 1, 2 and 3). A pure substance has a sharp, fixed melting point and boiling point, whereas an impure substance melts and boils over a range. So measuring the melting point (method 1) and the boiling point (method 2) and comparing each with the reference value will both reveal whether the substance is pure.
Paper chromatography (method 3) also works: a pure substance gives a single spot, while a mixture gives more than one. Because this substance is colourless, a locating agent is needed to make the spots visible, exactly as method 3 describes.
All three are valid purity checks, so the answer is A. Common error: some students reject chromatography for a colourless sample, forgetting that the locating agent is specifically there to solve that problem.
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