Final answer:
The Gram staining in Gram-positive bacteria is due to the presence of peptidoglycan in their thick cell walls, which allows them to retain the crystal violet stain.
Step-by-step explanation:
The Gram staining in Gram-positive bacteria is due to the presence of peptidoglycan in their cell walls. Gram-positive bacteria have a thick cell wall consisting of many layers of peptidoglycan, which is responsible for their ability to retain the crystal violet stain used in the Gram staining process. This is in contrast to Gram-negative bacteria, which have a thinner cell wall composed of a few layers of peptidoglycan and an outer membrane containing lipopolysaccharides and lipoproteins.
Therefore, the correct answer to the question is D. peptidoglycan.