Out of the given options, cryo-EM would be the preferred technique to determine the quaternary structure of a membrane protein. X-ray crystallography requires crystallization of the protein which can be challenging for membrane proteins. NMR is typically limited to smaller proteins and soluble proteins, and may not be suitable for determining the quaternary structure of larger, complex membrane proteins. Cryo-EM has become a powerful technique for determining the structure of membrane proteins and their complexes, even at low resolution. It allows the study of membrane proteins in their native environment, and can provide information about the quaternary structure, as well as interactions with other molecules such as lipids or ligands.