Cell Classification — What single criterion best distinguishes a prokaryotic cell from a eukaryotic cell?

Introduction:
Prokaryotes and eukaryotes are defined by internal compartmentalization. The most decisive differentiator is the presence of a membrane-bound nucleus in eukaryotes, which prokaryotes lack.

Given Data / Assumptions:

• Both groups contain DNA and ribosomes.
• Cytosol is universal in cellular life.
• Unicellularity vs multicellularity does not define the distinction (many eukaryotes are unicellular; some prokaryotes form multicellular-like colonies).

Concept / Approach:
Identify the necessary and sufficient structural hallmark that separates the two domains: the nuclear envelope and complex endomembrane system of eukaryotes.

Step-by-Step Solution:

Verification / Alternative check:
Taxonomic definitions: Eukarya possess a nuclear envelope; Bacteria and Archaea do not. This single structural criterion is standard in textbooks and courses.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming complexity equals multicellularity; overlooking that ribosomes and DNA are universal to cellular life.

Final Answer:
Presence of a membrane-bound nucleus.