D

Reasoning:  
• Proteoglycans are composed of a core protein to which multiple glycosaminoglycan (GAG) chains are covalently attached, giving a “bottle-brush” architecture [Silbert, 1982, PMID 7045244].  
• Each GAG repeat contains an amino sugar paired with a uronic acid (oxidized sugar acid), e.g., glucuronic or iduronic acid [Anower-E-Khuda, 2015, PMID 25325947].  
• The carboxyl and sulfate groups on these uronic acids confer high negative charge; electrostatic repulsion keeps the GAG chains extended and radiating outward from the core protein [Hua, 2021, PMID 34435181].  

Since all three individual statements (A, B, and C) are correct, option D (“All of these”) is the most accurate choice.