The correct answer is option B: Thermal radiation. A bolometer is an instrument used to measure thermal radiation. It detects and quantifies the intensity of infrared or heat radiation emitted by objects. When exposed to thermal radiation, a bolometer undergoes a change in electrical resistance, which is then measured to determine the intensity of the radiation. This principle allows bolometers to be used in various applications, including astronomy for studying celestial objects emitting infrared radiation, in thermal imaging devices for detecting heat signatures, and in industrial processes for monitoring temperature and thermal properties. Unlike instruments that measure thermal conductivity, intensity of sound, or specific heat, a bolometer specifically targets thermal radiation, making it a valuable tool in fields where precise measurement of heat energy is necessary for research, monitoring, and analysis.