Which cell organelle is concerned with glycosylation of protein :

Proteins made on ribosomes attached to the rough endoplasmic reticulum enter the ER lumen, where sugar chains are attached to them (N-linked glycosylation). This produces glycoproteins.
The Golgi apparatus further modifies these sugar chains.
Ribosomes only make the polypeptide, and peroxisomes and mitochondria are not involved in glycosylation.
So the endoplasmic reticulum is concerned with glycosylation of proteins.