Water on mountains starts boiling at a temperature less than 100°C. This phenomenon occurs because the atmospheric pressure is lower at higher altitudes compared to sea level. Boiling occurs when a liquid’s vapor pressure equals the surrounding atmospheric pressure. At higher elevations, the reduced atmospheric pressure lowers the boiling point of water. For instance, at an elevation of 2,000 meters (approximately 6,561 feet), water boils at about 93°C (199.4°F). This decreased boiling point has practical implications, such as affecting cooking times and methods in mountainous regions. Understanding this concept is essential for activities like mountaineering, high-altitude cooking, and certain industrial processes. Therefore, the correct answer is [A] Less than 100 °C. This reduction in boiling point due to lower atmospheric pressure at high altitudes is a well-established principle in physics and chemistry.