**Core Concept**
Protein folding is a crucial process in which a protein's linear sequence of amino acids is converted into its native three-dimensional structure. This process is essential for protein function, stability, and interactions. Chaperones and chaperonins play a significant role in facilitating protein folding, preventing protein aggregation, and maintaining cellular homeostasis.

**Why the Correct Answer is Right**
Protein folding is facilitated by molecular chaperones, which bind to the protein and prevent incorrect interactions, allowing the protein to fold correctly. Chaperones like Hsp70 and Hsp90 recognize and bind to specific sequences on the protein, preventing incorrect interactions and promoting proper folding. The correct folding of proteins is also facilitated by chaperonins, which form a cage-like structure around the protein, creating a confined environment that allows the protein to fold correctly. This process is essential for the proper functioning of proteins in various cellular processes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specifically relate to protein folding. While enzymes like peptidases do play a role in protein processing, they are not directly involved in the folding of proteins.
**Option B:** This option is incorrect as it refers to a process that occurs after protein folding, where the protein is modified by the addition of carbohydrate or lipid groups. This process is not directly related to the folding of proteins.
**Option C:** This option is incorrect as it refers to a process that occurs in the endoplasmic reticulum, where proteins are modified and folded in the presence of molecular chaperones. However, this is not the primary mechanism by which proteins fold.

**Clinical Pearl / High-Yield Fact**
Molecular chaperones play a crucial role in maintaining protein homeostasis and preventing protein aggregation, which is associated with various neurodegenerative diseases, including Alzheimer's and Parkinson's disease.

**Correct Answer:** D. Chaperones.