The correct answer is option 'D' - endoplasmic reticulum.

Explanation:
The endoplasmic reticulum (ER) is an organelle involved in the glycosylation of proteins. Glycosylation is the process by which sugar molecules (such as glucose, mannose, and galactose) are added to proteins, forming glycoproteins. This post-translational modification is essential for protein folding, stability, and function.

Here is a detailed explanation of why the endoplasmic reticulum is involved in glycosylation:

1. Structure of the Endoplasmic Reticulum:
The endoplasmic reticulum is a network of membranes found in eukaryotic cells. It consists of two regions: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The RER is studded with ribosomes, while the SER lacks ribosomes. The RER is primarily involved in protein synthesis and modification.

2. Role of the Rough Endoplasmic Reticulum (RER) in Protein Synthesis:
The RER is responsible for the synthesis of proteins destined for secretion, membrane insertion, or incorporation into other organelles. Ribosomes attached to the RER synthesize these proteins, which are then translocated into the ER lumen.

3. Protein Folding and Quality Control in the Endoplasmic Reticulum:
As proteins are synthesized within the RER, they are co-translationally translocated into the ER lumen. This allows for proper folding and modification of the proteins. The ER provides an environment conducive to protein folding and contains chaperone proteins that assist in the folding process.

4. Glycosylation in the Endoplasmic Reticulum:
During protein synthesis within the RER, certain proteins undergo glycosylation. This process involves the addition of sugar molecules to specific amino acid residues on the protein chain. The sugar moieties are usually attached to a lipid carrier molecule called dolichol phosphate, which is embedded in the ER membrane. The sugar moieties are transferred from the dolichol phosphate to the growing protein chain.

5. N-Linked and O-Linked Glycosylation:
There are two main types of glycosylation: N-linked and O-linked glycosylation. In N-linked glycosylation, the sugar moieties are attached to the nitrogen atom of specific asparagine residues in the protein chain. This process occurs within the ER. In O-linked glycosylation, the sugar moieties are attached to the oxygen atom of specific serine or threonine residues in the protein chain. O-linked glycosylation can occur in the ER or Golgi apparatus.

In summary, the endoplasmic reticulum, particularly the rough endoplasmic reticulum, is involved in the glycosylation of proteins. This organelle provides the environment and machinery necessary for protein synthesis, folding, and modification, including the addition of sugar molecules to proteins.