What might you use if you need to accurately measure the distance between two points in a lab?

Using a ruler is the most appropriate choice for accurately measuring the distance between two points in a lab setting. Rulers are specifically designed for this purpose and typically come with precise markings that allow for measurement in various units, such as centimeters or inches. This precision is crucial in laboratory settings where accuracy can significantly affect experimental outcomes.

In contrast, other options like a test tube rack, stirring rod, and thermometer serve different functions that do not align with the need for measurement of distance. A test tube rack is used to hold test tubes securely, a stirring rod is designed for mixing solutions, and a thermometer is used to measure temperature. None of these tools provide the capability to measure linear distance accurately.