Final answer:
While milliamperage, kilovoltage, and distance directly affect the exposure rate of the primary X-ray beam, field size determines the irradiated area and does not have a direct impact on the exposure rate.
Step-by-step explanation:
In the context of radiographic imaging, factors that influence the exposure rate of the primary beam include milliamperage (mA), kilovoltage (kV), and distance from the source. Milliamperage affects the number of electrons, thus impacting the amount of X-rays produced. Kilovoltage influences the energy of the electrons and thereby the penetration power of the X-rays. Distance follows the inverse square law, where increasing the distance from the X-ray source diminishes the exposure rate exponentially.
However, field size does not directly affect the rate of exposure of the primary beam, but instead affects the area being irradiated. Therefore, the correct answer to which of these factors does not affect the exposure rate of the primary beam is D. Field size.