Final answer:
A hot solid can produce a continuous spectrum, whereas cold gas at low density, hot gas at low density, and gas consisting entirely of molecules cannot.
Step-by-step explanation:
A continuous spectrum can be produced by a hot solid. When a solid is heated, it emits light at all wavelengths, resulting in a continuous spectrum. This is because the atoms in solids are closely packed, allowing for the absorption and emission of a wide range of wavelengths. On the other hand, a cold gas at low density, a hot gas at low density, and a gas consisting entirely of molecules would not produce a continuous spectrum, but rather line spectra with specific discrete wavelengths.