Which of the following indicates that the main rails have been deformed?

The presence of twisting, bowing, or bending of the frameworks is a clear indication that the main rails have undergone deformation. This type of distortion can occur due to excessive loads, impacts, or stress over time. It directly affects the structural integrity and functionality of the chassis, making it crucial for a chassis inspector to identify these signs during an examination.

In the context of the other options, while rust appearing on the surface can indicate corrosion and might affect the chassis over time, it does not directly signify deformation of the main rails. Similarly, wear and tear on the landing gear indicates a need for maintenance but does not explicitly reflect the condition of the main rails. Lastly, loose bolts on coupler plates could suggest assembly issues or a need for tightening but are not definitive evidence of deformation in the main rails themselves. Thus, the option regarding twisting, bowing, or bending directly relates to the structural issues that deformation of the main rails would cause.