Which laboratory tool is primarily used to prevent contamination and secure containers?

The tool primarily used to prevent contamination and secure containers is the rubber stopper. Rubber stoppers are designed to seal the openings of various laboratory glassware, such as flasks and test tubes, thus safeguarding the contents from exposure to air and potential contaminants. This creates a controlled environment for chemical reactions or biological samples, reducing the risk of contamination that might alter results or compromise safety.

In comparison, graduated cylinders are primarily used for measuring liquid volumes, and while they are important for conducting experiments, they do not serve to seal or secure containers. An eyewash station is a safety apparatus used for rinsing chemicals from the eyes, essential for personal safety but not related to preventing contamination in samples. Similarly, a first aid kit is meant for treating injuries in the lab, which does not address the need for sealing or securing containers in scientific work. Hence, rubber stoppers are the appropriate choice for the stated purpose.