Concept:The purity of a solid is best judged by its melting point because impurities change the melting behaviour in a characteristic way.
Explanation:A pure solid has a sharp, fixed melting point that matches the standard value for that substance.
When impurities are present, they cause melting point depression, meaning the solid melts at a lower temperature and over a wider range.
By comparing the observed melting point with the known standard value, one can easily determine the purity of the sample.
Boiling point is more useful for liquids, while conductivity and solubility are not reliable indicators of purity for a solid.
Therefore, a sharp melting point at the correct temperature confirms that the solid sample is pure.
Answer:B. melting point