What best describes a scenario in which a concept search might yield no results?

A concept search can yield no results, and the scenario where no matching documents are found in the index accurately reflects this situation. In this case, the absence of any documents that meet the specific criteria of the search means that the system has nothing to return as relevant results.

While options involving unsupported language or irrelevant stop words can affect the effectiveness of a search, they typically do not outright result in zero matches. Unsupported language might limit the search's scope, and stop words, though often filtered out of searches, may still allow for relevant terms to produce results if included. Therefore, the first choice most directly relates to the concept of the index having no documents to match against, leading to no search results being available.