The Farther Away a Galaxy Is, the More Its Light

The farther away a galaxy is, the more its light is redshifted, as observed from Earth. This relationship between redshift and distance is caused by

A) the Doppler shift of light leaving a moving object. More distant galaxies are moving faster through space, so their light is more strongly Doppler-shifted (redshifted) .
B) energy losses. The universe does not really expand; photons simply lose energy (wavelength lengthens) as they travel. Photons from more distant galaxies have traveled farther and so are more redshifted.
C) the expansion of space itself, which stretches the wavelength of the photon. The longer the time the photon has traveled, the more space has expanded and therefore the more the photon has been redshifted.
D) the gravitational redshift. Photons leaving a more distant galaxy have traveled farther through the galaxy's gravitational field, so they have lost more energy and are more redshifted.