I think the correct answer is "Archaebacteria". The kingdom which is completely composed of unicellular organisms that are prokaryotic is the kingdom of Archaebacteria. There are three major groups in this kingdom called methanogens, halophiles, and thermophiles.
<span> They're the main site of photosynthesis in autotrophs.
Hope this helps (:
-Payshence xoxo</span>
I think b. If that’s wrong plz say me to know it!