Models of three chemical structures are shown below.

To determine which statement is true regarding the three chemical structures (X, Y, and Z), we would need to analyze the characteristics of each model.

1. **Atom**: Typically represented by a single sphere or a small structure, often indicating an element.
2. **Molecule**: Represents a group of atoms bonded together (often depicted with several spheres connected).
3. **Extended Structure**: Often indicates a repeating unit or a large structure that extends in multiple directions, such as a crystal lattice or polymer.

Based on general conventions:
- If **Model X** looks like a simple sphere or atom, it would likely represent an atom.
- If **Model Y** appears to be interconnected and more complex (like a larger grouping or consistent pattern), it could represent an extended structure.
- If **Model Z** is a simpler structure but more complex than a single atom (but not extensive like Y), it would represent a molecule.

Without seeing the actual models, we cannot definitively conclude which statement is correct. However, based on typical descriptions:
- If we assume **X** is an atom, **Y** is a molecule, and **Z** is an extended structure, then one possible option could be **D**: "X represents an atom, Y represents a molecule, and Z represents an extended structure."

Please refer to the details of the models for accurate matching against these attributes to confirm or revise this interpretation.