Final answer:
Neutral atoms in the universe formed about a few hundred thousand years after the Big Bang, when the universe cooled to about 3,000 K. This occurred some 380,000 years post-Big Bang, transitioning the universe to a state where it became transparent to light. C is the correct answer.
Step-by-step explanation:
The formation of neutral atoms in the early universe marks a significant milestone in the history of the cosmos. This critical phase in the evolution of the universe—when it cooled enough for electrons and nuclei to combine into stable atoms—occurred approximately a few hundred thousand years after the Big Bang. Specifically, it was around 380,000 years old at a temperature of about 3,000 K that these conditions allowed for electrons to combine with protons, forming hydrogen atoms and creating a universe that became transparent to light for the first time.
During the universe's infancy, at roughly three minutes old and at a blistering temperature of about 900 million K, protons and neutrons began to combine, forming the nuclei of the simplest elements like hydrogen and helium, with traces of lithium.
These nuclei could not form earlier as any initial combinations were disrupted by the high-energy photons prevalent in such a hot, dense environment. Subsequent to nuclei formation, the universe continued to expand and cool until it finally reached the point where neutral atoms could be formed.
Therefore, the mentioned correct option in the final answer, corresponding to the time at which the universe cooled enough for neutral atoms to form, is c. a few hundred thousand years.