X represents the transition from Solid T to Gaseous T. This process is called sublimation*.

• X represents the transition from Solid T to Gaseous T. This process is called sublimation.
• Y represents the transition from Gaseous T to Liquid T. This process is called condensation.
• Z represents the transition from Liquid T to Solid T. This process is called freezing or solidification.

Therefore, X, Y, and Z respectively represent sublimation, condensation, and freezing.

Comparing this with the given options:
A. sublimation, condensation and freezing
B. sublimation, vaporization and solidification
C. freezing, condensation and sublimation
D. evaporation, liquefaction and solidification

The correct option is A.

The final answer is $\boxed{\text{A}}$