**Core Concept**
Prokaryotes are a type of cell that lacks a true nucleus and other membrane-bound organelles. They are characterized by their simple cell structure and are typically smaller than eukaryotic cells.

**Why the Correct Answer is Right**
Mitochondria are membrane-bound organelles responsible for generating energy for the cell through cellular respiration. They are a key feature of eukaryotic cells, which have a true nucleus and other membrane-bound organelles. Prokaryotes, on the other hand, lack mitochondria and rely on alternative mechanisms to generate energy. This is because prokaryotes have a simpler cell structure and do not require the energy-intensive process of cellular respiration that mitochondria facilitate.

**Why Each Wrong Option is Incorrect**
**Option A:** Cell membrane is a feature of both prokaryotic and eukaryotic cells, serving as the outermost layer of the cell.
**Option C:** Ribosomes are present in both prokaryotic and eukaryotic cells, responsible for protein synthesis.
**Option D:** Cell wall is a feature of prokaryotic cells, providing structural support and maintaining the cell's shape.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the absence of mitochondria in prokaryotes is a key distinguishing feature between prokaryotes and eukaryotes. This characteristic has significant implications for the metabolism and energy production of prokaryotic cells.

**✓ Correct Answer: B. Mitochondria**