Adding 1 mm of aluminum filtration to an x-ray beam results in:

Adding 1 mm of aluminum filtration to an x-ray beam primarily increases the average energy of the beam. Filtration serves to remove lower-energy photons from the x-ray beam, which are less likely to contribute to diagnostic imaging and more likely to be absorbed by the patient's skin.

As the low-energy photons are filtered out, the overall composition of the beam shifts to include more higher-energy photons, effectively raising the average energy. This is beneficial because higher-energy photons are more effective at penetrating tissue and resulting in better quality imaging while reducing the potential for unnecessary radiation exposure to the patient.

The filtration does not increase the patient skin dose; instead, it usually decreases it by eliminating low-energy photons that contribute to skin dose. Similarly, the overall output measured in mGya would not increase but could potentially decrease due to the reduced number of photons reaching the detector after filtering. Thus, the statement about the increase in average energy accurately reflects the purpose and effect of adding filtration to an x-ray beam.