Why can an insert band be faint and vector band bright in a restriction digest?

Correct answer: Insert size or amount may be lower

The governing concept is fluorescence or staining intensity as a function of DNA mass in a gel band. A band contains a total amount of DNA determined roughly by concentration multiplied by loaded volume. A small insert, or an insert present in lower copy or lower recovery, may contain less total DNA than the larger vector backbone. It therefore binds less stain and appears fainter, while the vector band appears brighter. Thus option A is correct. Faintness alone does not prove that the insert is protein or that the enzyme cut only the vector; those conclusions require additional controls or sequencing. The vector is DNA, not water.