Final answer:
To best protect a sculpture when not on display, a storage container is the most suitable choice as it safeguards against physical and environmental damages.
Step-by-step explanation:
The best way to protect your sculpture when it isn't on display is through the use of a d) Storage container. Unlike the other options, a storage container can offer a protective environment against physical damage, dust, and environmental factors such as humidity and temperature variations. Fixative sprays are more suitable for two-dimensional artworks like drawings or pastels, frames are for pictures or paintings, and page protectors are for documents or sheets of paper.