Final answer:
In a model of a silk molecule by a spider, each sphere represents C) an atom, showing the arrangement of atoms bonded together to form the molecule.
Step-by-step explanation:
In the model of a silk molecule made by a spider, each sphere represents an atom. This is because in molecular models, atoms are typically depicted as balls or spheres, with a cylindrical line representing the chemical bonds connecting them. These models help visualize the structure of molecules by showing how the atoms are bonded together. In the context of the model, the spheres are not representing the nuclei, ions, or compounds.