Restricting size of beam is done by:

 # Restricting size of beam is done by:

A. Collimator

B. Transformer

C. Tube head seal

D. Beam indicating device

The correct answer is A. Collimator.

A collimator is a metallic barrier with an aperture in the middle used to restrict the size of the x-ray beam and the volume of tissue irradiated. Round and rectangular collimators are most frequently used in dentistry. Dental x-ray beams are usually collimated to a circle 4 inches (7 cm) in diameter at the patient’s face. A round collimator is a thick plate of radiopaque material (usually lead) with a circular opening centered over the port in the x-ray head through which the x-ray beam emerges. Typically, round collimators are built into open-ended aiming cylinders. Rectangular collimators further limit the size of the beam to just larger than the x-ray film, further reducing patient exposure. Some types of film-holding instruments also provide rectangular collimation of the x-ray beam. 

Reference: White and Pharoah's Oral Radiology, 7th Edition