Nikon D750 User Manual page 235

Exposure Bracketing
A
The camera modifies exposure by varying shutter speed and aperture
(mode P), aperture (mode S), or shutter speed (modes A and M). If On is
selected for ISO sensitivity settings > Auto ISO sensitivity control
(0 136) in modes P, S, and A, the camera will automatically vary ISO
sensitivity for optimum exposure when the limits of the camera
exposure system are exceeded; in mode M, the camera will first use
auto ISO sensitivity control to bring exposure as close as possible to
the optimum and then bracket this exposure by varying shutter speed.
207