Explanation

The sentence describes the lamps in Pisa Cathedral moving back and forth in a regular, suspended motion. 'Swinging' means moving to and fro while hanging, matching 'dangling' in the context. 'Swings' is a finite verb form that breaks the noun construction, and 'swaying' suggests a gentler side-to-side motion, less apt for pendulum-like lamps. Hence 'swinging' is correct, option (B).