The spheres in this model represent atoms. The correct option is D.
Atom is the smallest unit of the element. Different elements have different size atoms and same element have same size atoms.
All matter consists of atoms. Atoms of the same element are the same in size and atoms of different elements are different. Atoms combine in whole-number ratios to form compounds.
And in any model, the atoms are represented as a sphere.
Thus, correct option is D.
Learn more about atoms.
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