Which letter on the diagram BEST represents that melting occurs​

In order from fastest to slowest, Steam > Liquid Water > Ice

Explanation:

As temperatures rise, particles are allowed to move faster and more freely. If you think about the phases of matter, the more compact something is (solids), the less the particles are able to move. Taking the temperate of ice into consideration, the cold further restricts fast movement of particles, whereas gas (especially steam in this case), in hot temperatures, is able to move quickly and freely in the atmosphere.