Which statement is INCORRECT regarding prokaryotic organism?

The correct option is: Nuclear region is defined.

To understand why this statement is incorrect, we can break down the key characteristics of prokaryotic organisms step-by-step:
1. Lack of a Defined Nuclear Region: Prokaryotic cells do not have a membrane-bound nucleus. Instead, their genetic material is concentrated in an poorly-defined region of the cytoplasm called the nucleoid, which lacks a surrounding nuclear membrane. Therefore, the statement that the nuclear region is defined is incorrect.
2. Primitive Structure: Prokaryotes represent a primitive form of life. They are structurally simpler and evolutionarily older than eukaryotic organisms.
3. Absence of Membrane-Bound Organelles: Unlike eukaryotes, prokaryotic cells do not contain membrane-bound organelles such as mitochondria, chloroplasts, lysosomes, or the Golgi apparatus. Thus, this statement is correct.
4. Single Chromosome: The genetic material in most prokaryotes consists of a single, circular chromosome. Thus, this statement is also correct.

Since the question asks for the statement that is INCORRECT, the correct choice is "Nuclear region is defined".