Final answer:
The energy stored in the unit volume of a stretched copper wire increases, as work is done to stretch the wire, storing more elastic potential energy.
Step-by-step explanation:
When a copper wire is stretched to increase its length, the energy stored in the unit volume of the wire changes. Assuming the stretching process is elastic and within the wire's limit of proportionality, the energy stored per unit volume, known as the elastic potential energy, increases.
The increase in length means the wire is under more tension, and it takes work to stretch the wire. This work is stored as potential energy within the stressed material.
When a copper wire is stretched, its length increases while its cross-sectional area decreases. However, the volume of the wire remains constant, which means that the energy stored in the unit volume of the wire remains constant as well.
Therefore, the correct answer is c. Unchanged.