Concept:The melting point of a solid is the most reliable indicator of its purity.
Explanation:A pure solid consists of a regular, uniform crystal lattice.
As a result, it melts completely at one fixed, sharp temperature.
If impurities are present, they disturb the crystal structure.
This lowers the melting point and makes it melt over a wide temperature range.
Therefore, a sharp and constant melting point confirms a pure solid sample.
Boiling point is more useful for liquids, not for testing solid purity.
Conductivity depends on the type of bonding and structure, not directly on purity.
Solubility is affected by many factors and is not a specific test for purity.
Hence, the best method is to determine the melting point.
Answer:B. melting point.