What Causes the Cosmological Redshift of Photons That Reach Earth

What causes the cosmological redshift of photons that reach Earth from distant galaxies?

A) The photons have traveled through space that has been expanding and their wavelengths have expanded with it, becoming redder.
B) The photons were emitted from the galaxies much earlier in time when the overall temperature of matter was much lower. Hence, the observed photons are redder the farther away from Earth they were produced.
C) The photons were emitted by objects that were moving rapidly away from Earth and thereby have been reddened by the Doppler effect.
D) The photons have moved from high-gravitational-field regions toward lower fields, thereby becoming reddened.