D

Step 1: Analyze the graph. The graph displays temperature on the y-axis and time on the x-axis. It shows a decrease in temperature over time, indicating a cooling process.
Step 2: Understanding the options:
- Adiabatic expansion of a gas (A): This involves a gas expanding with no heat exchange, but the temperature changes in a more complex manner.
- Isothermal expansion of a gas (B): This indicates a process where the temperature remains constant, which does not fit the graph.
- Change of state from liquid to solid (C): This typically would involve a temperature plateau at the freezing point; this graph does not show such behavior.
- Cooling of a heated solid (D): The continuous decrease in temperature over time aligns with a cooling process.
Conclusion: The graph most accurately represents the cooling of a heated solid. Therefore, the correct answer is D.