The approximate temperature of the cosmic microwave background radiation is 2.7 K. More precisely, measurements give a present-day temperature of about 2.725 K, just a few degrees above absolute zero.
The cosmic microwave background, or CMB, is relic radiation released when the early universe cooled enough for electrons and atomic nuclei to combine. This occurred roughly 380,000 years after the Big Bang, allowing photons to travel freely through space. As the universe expanded, those photons were stretched to longer microwave wavelengths and cooled to their present temperature.
The CMB is nearly uniform across the sky, but tiny temperature variations reveal differences in the density of the early universe. Those fluctuations later helped grow into galaxies and large-scale cosmic structure. Satellite missions including COBE, WMAP, and Planck measured the radiation and its variations with increasing precision.
The other choices are not the standard temperature of the CMB. A value such as 1.9 K can describe other astrophysical radiation fields or approximations in different contexts, but it is not the accepted present-day CMB temperature.