Which of the following factors primarily determines the formation of quaternary protein structure in multi-subunit complexes?

The quaternary structure of multi-subunit protein complexes is primarily stabilized by hydrophobic interactions between nonpolar regions of the subunits. While other interactions like disulfide bonds play roles in protein structure, they are more often associated with other structural levels or individual subunit stability. Peptide bonds are involved in the primary structure, and glycosylation and carboxylation are related to post-translational modifications rather than quaternary structure.