The principal reason for placing an aluminium filter in
the primary beam of radiation is to:
(A) Reduce exposure time (B) Decrease development time of the films (C) Reduce radiation to the skin of the patient (D) Obtain greater definition of the images of teeth
(C) Although an x-ray beam consists of a continuous spectrum of x-ray photon energies, only photons with sufficient energy to penetrate through anatomic structures and reach the image receptor (digital or film) are useful for diagnostic radiology. Low-energy photons that cannot reach the receptor contribute to patient risk but do no offer any benefit. Consequently, it is desirable to remove these low-energy photons from the beam. This removal can be accomplished in part by placing a metallic disk (filter) in the beam path. 
A filter preferentially removes low-energy photons from the beam, while allowing high energy photons that are able to contribute to making an image to pass through. 
Ref: Oral Radiology, Principles and Interpretation / Stuart C. White, Michael J. Pharoah - 7th ed - pg - 10