Final Answer:
The normal transition temperature, T_trs, is the temperature at which B) A substance undergoes phase transition.
Step-by-step explanation:
The normal transition temperature, T_trs, specifically refers to the temperature at which a substance undergoes a phase transition.
This transition could involve changes from a solid to a liquid, liquid to gas, or other phase transformations.
It is a critical point where the substance experiences a shift in its physical state.
In the realm of material science and thermodynamics, understanding T_trs is essential for predicting and comprehending the behavior of substances under different temperature conditions.
It is not associated with the specific states mentioned in options A, C, or D.
Option A talks about a substance being in a gaseous state, which is not necessarily related to T_trs. Option C mentions superconductivity, which is a distinct property not tied to normal phase transitions.
Option D refers to the melting point, which is a specific phase transition but not synonymous with the broader concept represented by T_trs.
Therefore, the correct answer is B) A substance undergoes phase transition.