Where did CMB radiation originate?

(a) It is the radiation from the electroweak phase transition when the weak force was spon-
taneously broken by the Higgs mechanism.
(b) It is radiation from supernovas in the early Universe that produced the heavier elements.
(c) The radiation is the photons that were made at the very beginning of the Big Bang.
(d) It is radiation leftover from the time period where electrons combined with protons to
form neutral hydrogen and hence no longer were in equilibrium with light.