Solid melting due to heat input

Steps:

• The substance has a fixed shape, indicating it is a solid.
• Supplying thermal energy heats the solid.
• Weakening molecular attractions signals a phase change.
• This occurs at the melting point, transitioning solid to liquid.

Why D is correct:

• At the melting point, added heat energy breaks intermolecular forces in the solid lattice, allowing molecules to move freely as a liquid, per phase transition principles.

Why the others are wrong:

• A: Gases lack fixed shape and do not boil as solids.
• B: Liquids have no fixed shape and boiling involves liquid-to-gas change.
• C: Evaporation occurs in liquids, which lack fixed shape.

Final answer: D