Plot showing which phase of a substance is most stable at a given combination of temperature and pressure.

• A. Phase
• B. Phase Diagram
• C. Phase Boundaries
• D. Triple Point

Answer: (B)

Phase diagrams map how stable forms of matter change with temperature and pressure. Each region on the plot shows which phase (solid, liquid, or gas) is most stable under those conditions, so you can read off the prevailing phase for any given T and P. This is exactly what the question is asking: which form remains stable at a particular temperature–pressure combination. The diagram is built with phase boundaries—lines where two phases are in equilibrium—so the regions are divided by these lines. The triple point is just a special coordinate where three phases coexist, not the whole map of stability. That’s why this option is the best fit: it represents the complete map of phase stability across temperature and pressure.