What Is the Isotropy (Or Horizon)problem

What is the isotropy (or horizon) problem?

A) The universe is not as uniform as the standard cosmological model predicts it should be.
B) If the universe expanded at the rate originally predicted, then the parts we see in opposite directions would never have been in equilibrium with each other and should not exhibit the degree of isotropy we observe.
C) Without modification, the standard theory suggests that an observer on a distant galaxy would not be able to see our Galaxy because of the intense cosmological redshift.
D) As we observe galaxies on the "horizon," the farthest distance obtainable, the standard theory predicts that the recessional velocity should approach the speed of light.