Final answer:
During a phase change, the graph of temperature versus heat added will show a constant temperature despite energy being added, corresponding to the substance undergoing the change of state. The correct multiple-choice option is (a).
Step-by-step explanation:
During a phase change, the temperature of a substance does not increase or decrease despite the input or removal of heat. This is because the energy is used for the change of state, such as melting or boiling, without changing the temperature.
Therefore, the graph of temperature versus heat added during a phase change would show a horizontal line or plateau, indicating that the temperature remains constant until the phase change is complete. The correct multiple-choice option is (a).