None of the above, but a, b and c are each partly right. d has nothing at all to do with it.
The red shift is caused by the lengthening of the wavelength of light as it travels away from us. The Doppler Effect is the same effect for sound waves, and why trains, cars and sirens are high pitched as they approach and lower pitched as they recedes, but Doppler is for sound, not light.
Aging of light has some correctness to it, because the longer that light travels away from us the farther away it is, and the faster it recedes and the greater the red shift (Think of a balloon - points farther way expand outward faster than those closer together as you blow it up.)
Space is expanding and light waves are red shifted, but not just because space is expanding, but because light sources are traveling away from us. this is sort of saying the same thing, but not exactly.