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

The correct answer is Mesosome

Explanation:

• Mesosome is an infolding of the plasma membrane found in prokaryotic cells, especially in bacteria.
• Mesosomes are invaginations of the plasma membrane.
• These extensions are in the form of vesicles, tubules and lamellae. They help in cell wall formation, DNA replication and distribution to daughter cells.
• They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.

Other Options:

• Chromatophores: These are membranous structures found in photosynthetic prokaryotes like cyanobacteria. Their primary role is in photosynthesis.
• Cristae: Cristae are folds of the inner mitochondrial membrane found in eukaryotic cells. They are involved in oxidative phosphorylation and ATP production. 
• Endoplasmic Reticulum: The endoplasmic reticulum is an organelle exclusive to eukaryotic cells and is involved in protein and lipid synthesis. Prokaryotic cells do not have this organelle.