Answer: A thermometer
Explanation: The best tool for measuring temperature change in this scenario would be a thermometer. A thermometer is specifically designed to measure temperature and can accurately indicate changes in temperature when substances are mixed or undergo reactions. When sodium hydroxide (NaOH) is dissolved in water, it is an exothermic reaction, meaning it releases heat. By using a thermometer, Thomas can monitor and measure the temperature change that occurs as sodium hydroxide dissolves in water. It is important to select a suitable thermometer that can accurately measure the desired temperature range and ensure that it is calibrated correctly for accurate readings. Additionally, it is advisable to use a thermometer that is specifically designed for liquid measurements, such as a liquid-in-glass thermometer or a digital thermometer with a suitable probe for liquid measurements. These types of thermometers provide reliable temperature readings and are commonly used in scientific experiments and laboratory settings to measure temperature changes accurately.