Final answer:
The best way to brighten the image without affecting the beam intensity is to d)decrease the rejection, which increases the number of reflected sound waves accepted by the machine, thus brightening the image.
Step-by-step explanation:
The question relates to how one can brighten the image on a monitor without affecting the beam intensity of a device that uses sound waves, such as an ultrasound machine. The correct option that would brighten the image, without changing the beam intensity, is (D) Decrease rejection.
Decreasing the rejection would increase the number of reflected sound waves that are accepted by the machine, which would brighten the display image without increasing intensity of the transmitted beam. Adjustments to amplitude or frequency that alter the beam itself are not suitable, as they would change the beam's intensity. Options involving adjustments to monitor brightness or the properties of the transmitted sound would also not achieve the desired outcome without altering the beam.