The portion A is which represents the phase in which water molecules have the least amount of kinetic energy.
The molecules of a body are constantly moving at different speeds; this movement is called thermal motion. For this reason each molecule has an associated kinetic energy, which is related to the temperature that has the body.
For example, the average kinetic energy to the molecules of a gas, can be expressed by the following expression:
Ek = 3/2KT
K→ is the Boltzmann constant.
T → is body temperature
Thus, to the extent that the body temperature decreases, the average kinetic energy of molecules is reduced and therefore their speeds; the translational movement disappears and the molecules are vibrating in fixed positions in a crystal lattice.