Which option defines a thermostable substance?

A thermostable substance is defined as one that is not easily altered by heat, which means it maintains its structure and activity even when exposed to elevated temperatures. This property is particularly essential in various fields, such as biochemistry and pharmaceuticals, where the stability of compounds at different temperatures is critical.

Substances that are thermostable can be used in applications where they may be subjected to heat during processing, storage, or shipping without losing efficacy or undergoing detrimental changes. For instance, certain enzymes and vaccines are designed to withstand higher temperatures to ensure they remain functional and safe for use.

The other options suggest properties that would indicate a lack of stability when exposed to heat. A substance that requires refrigeration for stability indicates that it is sensitive to temperature changes and could degrade or remain inactive outside of cold conditions. Conversely, a substance that is only stable at low temperatures would also not be considered thermostable, as it would not hold up well under heat. Lastly, a substance that increases reactivity with heat suggests that it could become more unstable or engage in unwanted reactions, which conflicts with the definition of thermostability. Thus, the accurate definition of a thermostable substance lies in its resistance to change when exposed to heat.