Why Did the Era of Nuclei End When the Universe

Why did the era of nuclei end when the universe was about 380,000 years old?

A) All the free particles had combined to form the nuclei of atoms.
B) The universe had expanded and cooled enough for stable, neutral atoms to form.
C) Atomic nuclei were finally able to escape the plasma of the early universe.
D) Photons were finally able to escape the plasma of the early universe and were no longer available to produce hydrogen and helium nuclei.
E) The nucleosynthesis era that produced the nuclei heavier than helium ended.