In quaternary structure, subunits are linked by:: (A) Peptide bonds, (B) Disulphide bonds, (C) Covalent bonds, (D) Non-covalent bonds
Answer is D. The forces that keep the quaternary structure are hydrogen bonds, electrostatic bonds, hydrophobic bonds and van der Waals forces.
Reference: Vasudevan 7th  ed, pg 38