Step 1: Concept
In thermodynamics, the entire universe is divided into two parts: the system and the surroundings.
Step 2: Meaning
The "system" is not restricted by state of matter (solid, liquid, or gas).
Step 3: Analysis
A system is simply the specific portion of matter or space within a defined boundary that we choose to study or analyze.
Step 4: Conclusion
Therefore, a system is that part of the universe selected for our consideration or study.
Final Answer: (A)