c. relief sculpture
a. Incorrect. Though associated with architecture and sculpture, installations are not specifically shallow sculptures often used as wall decorations.
b. Incorrect. Kinetic sculptures are forms that can float free in space.
c. Correct. Relief sculptures, often used as wall decorations, are a shallow projection from a flat surface.
d. Incorrect. Sculpture in the round cannot be part of a flat surface, as the viewer can walk all the way around it.
e. Incorrect. Earth art breaks out of buildings and is built in direct relation with nature.