Commonly used collimating device: (A) Aluminium filter, (B) Lead diaphragm, (C) Molybdenum cup, (D) Tungsten filament
Answer is B. The collimating device often used is a lead diaphragm with a circular aperture for circular PIDs or a rectangular aperture for rectangular PIDs and the lead-lined PID. 
Frommer’s Radiology for Dental Professional, pg-22