Final answer:
A heating mantle should be used to reflux both volatile and non-volatile substances because it provides controlled and even heat distribution without an open flame, which is especially important for the safety and consistency when handling volatile substances with low boiling points.
Step-by-step explanation:
When dealing with the reflux of substances in a chemistry lab setting, the heat source selected must be suitable for the properties of the substance being heated. In the case of a volatile substance, which has a low boiling point and relatively weak intermolecular interactions, an open flame could be too intense and pose a risk of ignition. Therefore, a heating mantle is preferred for volatile substances as it provides a more controlled and even heat distribution without an open flame. On the other hand, a non-volatile substance, which has a high boiling point and relatively strong intermolecular interactions, could also be heated using a heating mantle to prevent overheating and provide stable temperature control. Consequently, the correct answer would be option b) Heating mantle for both volatile and non-volatile substances.