Purification raises melting point of solids via crystallisation Steps:

• Impure samples melt at lower temperatures due to impurities depressing the melting point.
• Purification removes impurities, increasing the melting point to the pure compound's value.
• Compound X is a solid (has melting point), so crystallisation is the appropriate purification method.
• After crystallisation, the melting point rises above 120°C, to 125°C for the pure compound.

Why C is correct:

• Crystallisation selectively purifies solids by solubility differences, raising the melting point from 120°C to 125°C as per melting point depression principle.

Why the others are wrong:

• A: Crystallisation is correct method, but 115°C is lower, contradicting melting point elevation upon purification.
• B: Distillation suits liquids by boiling point, not solids; 125°C temperature is correct but method is wrong.
• D: Distillation is unsuitable for solids; 115°C is also too low for pure compound.

Final answer: C