Answer:
The correct answer is option B. - mercury.
Explanation:
The thermometer is a device that helps in measuring the temperature or the temperature gradient or any matter or the region. Thermometer normally has a glass tube with marking for temperature and a sensing agent such as mercury that show change according to the temperature rise or fall
The standard lab thermometer has mercury in it that rises if the temperature rises and reaches the temperature marking according to the temperature.
Thus, the correct answer is option B. mercury.