At an age of 380,000 years, the temperature of the universe had fallen to 3000 k, and electrons could then combine with protons to produce hydrogen gas instead of roaming freely through space.
what major transition occurred as a consequence of this change in the universe at this time?

a) the present laws of physics were applicable to the properties of the universe time.
b) the universe became transparent to light for the first time.
c) nuclear fusion no longer occurs below this temperature, and so, general fusion throughout the universe would have ceased.
d) the universe would have lost its electrical charge suddenly to become elect.

The major transition occurred as a consequence of this change in the universe at this time is that  b)The universe became transparent to light for the first time.

Explanation:

For the first 380,000 years or so, the universe was essentially too hot for light to shine. The heat of creation smashed atoms together with enough force to break them up into a dense plasma, an opaque soup of protons, neutrons and electrons that scattered light like fog. Then 380,000 years after the Big Bang, matter cooled enough for atoms to form during the era of recombination, resulting in a transparent, electrically neutral gas.

This set loose the initial flash of light created during the Big Bang, which is detectable today as cosmic microwave background radiation. However, after this point, the universe was plunged into darkness, since no stars or any other bright objects had formed yet.

a and b sometimes have a positive value