Final answer:
The radiographic sign of lingual displacement of the root of an intruded primary incisor is foreshortening, where the tooth root appears shorter on the X-ray due to the projection angle.
Step-by-step explanation:
The radiographic finding that supports the diagnosis of lingual displacement of the root of an intruded primary incisor is B. Foreshortening. When an incisor is intruded or displaced lingually, the X-ray projection shortens the image of the root on the radiograph, creating a foreshortened appearance. This happens because the X-ray beam is projected at an angle that is more perpendicular to the axis of the tooth, which compresses the image of the tooth root on the film.
On the other hand, elongation occurs when the tooth root appears stretched on the radiograph due to the X-ray beam being angled more parallel to the tooth axis. Increased opacity refers to the tooth or surrounding structures appearing denser and whiter on the radiograph, which is not specifically related to the position of the tooth root. A widened periodontal ligament space may indicate trauma or infection but does not specifically indicate the lingual displacement of an intruded tooth.