**Core Concept**
N-glycosylation is a post-translational modification process that adds carbohydrate molecules to proteins, which is crucial for protein stability, folding, and function. This process involves the transfer of a glycosyl group from a donor molecule to the asparagine residue of a target protein.

**Why the Correct Answer is Right**
The N-glycosylation process begins in the endoplasmic reticulum (ER), where the oligosaccharyltransferase complex catalyzes the transfer of a glycosyl group from a dolichol-linked oligosaccharide to the asparagine residue of the target protein. This process is mediated by the enzyme oligosaccharyltransferase, which is embedded in the ER membrane. The resulting glycoprotein is then further modified in the ER and Golgi apparatus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because N-glycosylation does not occur in the Golgi apparatus, which is involved in O-glycosylation and further modification of N-glycans.
**Option B:** This option is incorrect because the initial steps of N-glycosylation occur in the ER, not in the cytosol.
**Option C:** This option is incorrect because the final steps of N-glycosylation, including the removal of the mannose-6-phosphate marker and the addition of sialic acid, occur in the Golgi apparatus, but not all steps occur here.

**Clinical Pearl / High-Yield Fact**
N-glycosylation is essential for the proper folding and function of many proteins, including those involved in the immune response, such as antibodies, and those involved in cell-cell interactions, such as selectins.

**Correct Answer:** D.