Which of the following is a use of ultraviolet light?

Ultraviolet light is known for its germicidal properties, making it effective at killing bacteria and other microorganisms. This application takes advantage of UV light's ability to damage the DNA of bacteria, rendering them unable to reproduce and effectively neutralizing them. This characteristic is widely utilized in various settings, such as in water purification systems, sterilization of medical equipment, and disinfection of surfaces in healthcare environments.

While other options mentioned have their own distinct uses, they are not typically associated with ultraviolet light. For instance, medical imaging often relies on techniques like X-rays and MRI, detecting broken bones also primarily uses X-ray technology, and navigation underwater will commonly utilize sonar or other methods rather than ultraviolet light. Thus, using ultraviolet light for killing bacteria stands out as a well-established and practical application.