Multi-center, open-label, randomized (for dose schedule) clinical study with dose escalation (Phase 1) and expansion (Phase 2) stages to independently assess the maximum tolerated dose (MTD,) efficacy, and safety of selinexor + pomalidomide + dexamethasone (SPd), selinexor + bortezomib + dexamethasone (SVd), selinexor + lenalidomide + dexamethasone (SRd), and selinexor + daratumumab + dexamethasone (SDd) in patients with relapsed/refractory multiple myeloma (RR MM).