A specialized membranous structure in a prokaryotic cell which helps in cell wall formation, DNA replication and respiration is:

The correct option is Mesosome.

Step-by-Step Explanation:

1. Understanding Prokaryotic Membranous Structures:
Prokaryotic cells lack membrane-bound organelles (like mitochondria, endoplasmic reticulum, or Golgi apparatus) which are typically found in eukaryotic cells. However, they possess specialized cell envelope structures and membranous extensions to perform vital cellular functions.

2. What is a Mesosome?
A mesosome is a specialized, differentiated form of the cell membrane characteristic of prokaryotes. It is formed by the extensions of the plasma membrane into the cell. These extensions are usually in the form of vesicles, tubules, and lamellae.

3. Functions of Mesosomes:
Mesosomes play multiple critical roles in the life cycle and metabolism of a prokaryotic cell:

• Cell Wall Formation: They help in synthesizing and directing materials needed for the bacterial cell wall.
• DNA Replication and Distribution: They assist in DNA replication and the subsequent distribution of chromosomes to daughter cells during cell division (binary fission).
• Respiration and Secretion: They increase the surface area of the plasma membrane, packing enzymatic machinery responsible for cellular respiration (similar to mitochondria in eukaryotes) and active secretion processes.

4. Evaluating the Other Options:

• Chromatophores: These are pigment-containing membranous structures found in some photosynthetic prokaryotes (like cyanobacteria) that help in photosynthesis, but they are not primarily involved in cell wall formation or DNA replication.
• Cristae: These are the foldings of the inner membrane of mitochondria. Since mitochondria are eukaryotic organelles, cristae do not exist in prokaryotic cells.
• Endoplasmic Reticulum: This is a complex membrane network found exclusively in eukaryotic cells.

Therefore, the membranous structure in prokaryotes responsible for cell wall formation, DNA replication, and respiration is the Mesosome.