Final answer:
After the evaporation of a liquid containing dissolved solid salt (NaCl) crystals, the residue left is salt crystals.
Step-by-step explanation:
In the context of evaporation and the images provided, when liquid evaporates at a temperature below its boiling point, it leaves behind any dissolved substances in solid form. If the right-side picture the student is referring to involves the evaporation of a liquid that contained dissolved solid salt (NaCl) crystals, then what is left behind after the evaporation process would be salt crystals.
For example, when seawater evaporates, which contains salts, it leaves behind salt deposits because the water vaporizes and the salt does not; it remains as a solid residue. This is what occurs in natural processes like the formation of salt flats from evaporated seas or lakes. Therefore, if this scenario matches the details of the right-side picture that describes evaporation, then the correct answer to what is left behind after evaporation is (a) salt crystals.