Size-reduction equipment can be grouped by the physical forces each machine mainly relies on: compression, impact, attrition, or shear, and some mills combine more than one of these.
Roller mills lean on compression and shear as material is drawn between rollers. Colloid mills also lean on shear, forcing material through a narrow rotor-stator gap to break droplets down for emulsions. Hammer mills rely almost entirely on impact, swinging hammers striking particles at speed, with very little attrition involved.
The fluid energy mill stands apart because it uses high-velocity compressed air jets to send particles crashing into each other and into the walls of the chamber. That collision supplies the impact, and as the particles continue to swirl and rub against each other in the turbulent air stream, they also grind each other down, which supplies the attrition. This dual action is what lets a fluid energy mill reach very fine particle sizes, including micronisation.
Since impact and attrition working together is the defining mechanism only of the fluid energy mill, that is the correct answer.