If the universe were to suddenly begin shrinking rather than continuing to expand, it would have a significant effect on the cosmic microwave background radiation (CMB).
The CMB is the afterglow of the Big Bang and is observed as a nearly uniform background radiation in all directions. It is thought to have been emitted when the universe was about 380,000 years old and had cooled enough for neutral atoms to form.
If the universe were to suddenly begin shrinking, the photons in the CMB would lose energy as they travel through the contracting space. This would cause the CMB radiation to shift to shorter wavelengths, which is known as blue-shifting.
Therefore, the correct answer is B. It would blue-shift.