Which laboratory equipment is used primarily to hold or support test tubes during experiments?

The test tube rack is specifically designed to hold and support multiple test tubes securely during experiments. This equipment is crucial for maintaining organization in the laboratory, allowing for safe and efficient handling of samples. By keeping the test tubes upright, the rack helps prevent spills and cross-contamination, while also enabling easy access for observations and measurements.

The other options serve different purposes; for example, a wash bottle is used for dispensing water or other liquids, a spatula is used for transferring solids or powders, and a ruler is employed for measuring lengths or distances. Thus, the test tube rack is the most suitable choice for the function of holding test tubes.