Answer
Letter A = FreeziExplanation
The state of the matter depends upon the temperature. Each state (solid, liquid, and gas) has its own unique set of physical properties.
Common changes of the state include melting (solid to liquid), freezing (liquid to solid), sublimation (solid to gas), deposition (gas to solid), condensation (gas to liquid), and vaporization (liquid to gas).
From the given graph, A indicates a change from liquid to solid, hence, the letter A is the Freezing point.