Which example represents an explicit location in a query?

The choice of "libraries in San Francisco" represents an explicit location in a query because it clearly specifies a geographic area along with the subject of interest, which in this case is libraries. Including "San Francisco" directs the search or inquiry to a specific city, enabling the retrieval of information that is geographically relevant. This explicitness ensures that results will pertain specifically to libraries located in that city, which is what makes it precise and definitive in terms of location.

In contrast, the other examples are more general and vague regarding location. "Popular libraries" does not specify where these libraries are located, and could refer to libraries in any location. "Libraries nearby" also lacks a specific geographic reference, as 'nearby' could pertain to any area relative to the user's current location. "All about libraries" is the broadest of the options, failing to mention any location, which makes it the least explicit in terms of geographic context. Thus, the specificity of "libraries in San Francisco" is what makes it the correct choice for representing an explicit location in a query.