Which of the following is a sign of a dull dental instrument?

The indication that a dental instrument is dull can indeed be inferred from the outcome of sliding over a test stick. When a dull instrument is used, it often fails to grasp or cut effectively, resulting in it sliding without engaging the surface of the test stick properly. A sharp instrument, on the other hand, would penetrate the test stick and produce a tactile feedback consistent with effective cutting.

The other options highlight characteristics of sharp instruments. A sharp edge would be indicative of an instrument that can efficiently engage with and manipulate hard or soft tissues. Reflecting no light suggests poor finish or dullness but does not specifically confirm that the instrument is dull in performance terms—it's more about the quality of the surface finish rather than its functional sharpness. Lastly, high tactile sensitivity indicates a good handling characteristic of sharp instruments, which allows clinicians to detect subtle changes in tissue consistency and other sensations, a quality that would diminish with a dull instrument.

So, the behavior of a dull instrument, particularly its inability to engage effectively with the test stick, serves as a clear and practical sign in a clinical setting.