Step 1: Concept
An isomorphous system is one where the two components are entirely soluble in each other in both the liquid and solid states.
Step 2: Meaning
This typically occurs in systems like Copper-Nickel where the Hume-Rothery rules are satisfied (similar atomic size, crystal structure, and electronegativity).
Step 3: Analysis
"Complete solubility" means only one phase exists across the entire composition range at appropriate temperatures.
Step 4: Conclusion
An isomorphous binary phase diagram represents complete liquid and complete solid solubility.
Final Answer: (D)