Why Does the Observable Universe Have an "Edge

Why does the observable universe have an "edge"?

A) Astronomers cannot see any farther out into space than the distance that light has traveled over the lifetime of the universe.
B) The density of neutrinos at the "edge" becomes so large that photons cannot penetrate this barrier, and this prevents observers from seeing beyond this point.
C) Absorbing matter prevents observers from seeing beyond a certain distance.
D) There are so many galaxies in the universe that every line of sight eventually hits a galaxy, stopping observers from seeing any farther.