Explanation

The central region of a prokaryotic cell holding its DNA is the nucleoid, since bacteria and archaea lack a membrane-bound nucleus. A true nucleus (option A) enclosed by a nuclear membrane occurs only in eukaryotic cells. The nucleolus (option C) is a structure inside the eukaryotic nucleus, and plastids (option D) are plant organelles. Hence the nucleic-acid-containing central region of a prokaryote is the nucleoid.