This study will independently assess the efficacy and safety of four combination therapies for the treatment of patients with relapsed/refractory multiple myeloma (RR MM): selinexor + pomalidomide + dexamethasone (SPd), selinexor + bortezomib + dexamethasone (SVd), selinexor + lenalidomide+ dexamethasone (SRd), and selinexor + daratumumab + dexamethasone (SDd). The abbreviations for combination treatments have been revised to use V (Velcade) for bortezomib and R (Revlimid) for lenalidomide.