What feature on an endless loop sling indicates if it has been stretched or overloaded?

The correct choice is the tattle-tail. This feature is specifically designed to provide a visual indication of whether the sling has been subjected to excessive stress, such as being overloaded or stretched. The tattle-tail typically consists of a colored or contrasting piece of material that becomes visible or changes in appearance when the sling experiences conditions that could compromise its integrity.

In the context of rigging and lifting operations, recognizing whether equipment has been overstressed is crucial for safety and performance. The ability to easily identify a tattle-tail allows riggers and operators to assess the condition of the sling at a glance, ensuring that any potential issues can be addressed before further use.

The other options, while related to rigging or sling features in some respect, do not specifically serve the same purpose as the tattle-tail. For instance, an indicator line or load marker may provide different types of information about the sling's capacity or load characteristics, but they do not directly indicate overstretch or overload conditions like the tattle-tail does. A measurement tape is generally more focused on length or size rather than the condition after loading, so it does not fulfill this particular safety-check function.