What Is the "Cosmological Redshift

What is the "cosmological redshift"?

A) the stretching of the wavelengths of photons as they travel through expanding space
B) the stretching of the wavelengths of photons by the Doppler shift because they are emitted by galaxies that are moving away from us
C) the loss of energy from photons interacting with virtual particles in the vacuum, resulting in the wavelength of the photons gradually increasing as they travel toward us through space
D) the stretching of the wavelengths of photons as they pass through absorbing matter in galaxies between us and the emitting galaxy