During a C-100 Freight Car Inspection, what indicates a problem with the carbody?

A carbody that is sagging downward is a clear indication of a structural problem. This sagging may result from various issues, such as weakened support structures, damage from impacts, or corrosion that compromises the integrity of the car. Such problems can affect the safety and operational efficiency of the freight car, potentially leading to significant problems during transport if not addressed.

In contrast, the other options - a perfectly positioned car, all doors being secured, and having no objects dragging - do not indicate issues with the carbody itself. A well-positioned car or secure doors are signs of a car that is in good order, while the absence of dragging objects typically pertains to safety during transit rather than structural integrity. Thus, sagging is indeed the most direct indication of a concern regarding the carbody.