Which characteristic defines a fixed-temperature heat detector?

A fixed-temperature heat detector is defined by its operational principle, which relies on two electrical contacts that close when a specific temperature threshold is reached. This type of detector typically has an element that responds to rising temperatures, and once the predetermined temperature is surpassed, the contacts close and activate the fire alarm system.

This characteristic makes option C the right choice, as it accurately describes the mechanism by which this type of heat detector functions. It is specifically designed to respond to heat alone, without considering other environmental factors such as sound, smoke, or humidity.

The other options do not pertain to the function of a fixed-temperature heat detector. For instance, the activation based on sound levels pertains more to sound detection systems rather than heat detection. Using chemical reactions for smoke detection refers to smoke detectors, which are fundamentally different from heat detectors. Lastly, monitoring humidity levels is not a function of heat detectors, as humidity does not directly trigger an alarm related to temperature changes.