A Study of Selinexor (Seli) + Low-dose Dexamethasone (LDD) in Penta-refractory Multiple Myeloma (MM), Seli and Bortezomib + LDD in Triple-class Refractory MM.

* Age greater than or equal to (>=)18 years at the time of signing informed consent.

* Written informed consent in accordance with federal, local, and institutional guidelines.

* Measurable MM based on IMWG guidelines as defined by at least one of the following:

1. Serum M-protein >= 0.5 gram per deciliter (g/dL) by serum protein electrophoresis (SPEP) or, for Immunoglobulin (Ig) A myeloma, by quantitative IgA.

2. Urinary M-protein excretion >= 200 mg/24 hours.

3. Free light chain (FLC) >= 100 milligram per liter (mg/L), provided that the FLC ratio is abnormal.

* Only for arms Sd-40 BIW, Sd-100 QW and Sd-80 BIW prior to protocol version (PV) 5.0: Participants must have relapsed or refractory multiple myeloma (RRMM) and have previously received at least 4 anti-MM prior therapies and have MM that is refractory to previous treatment with at least 2 proteasome inhibitors (PIs), at least 2 immunomodulatory agent (IMiDs), and 1 anti-cluster of differentiation (CD38) monoclonal antibody. Refractory is defined as lesser than or equal to (<=) 25 percent (%) response to therapy, or progression during therapy or progression within 60 days after completion of therapy.

* Only for Arms Sd-40 BIW and Sd-100 QW as of PV 5.0: Participants must have RR MM and have been previously treated with >=3 anti-MM therapies (with exposure to at least 2 PI drugs, at least 2 IMiDs, and 1 anti-CD38 monoclonal antibody), and be refractory to at least 1 drug of each class (PI/IMiD/anti-CD38). Refractory is defined as <=25% response to therapy or progression during therapy or progression within 60 days after completion of therapy.

* Only for arm SVd: Participants must have previously received 1 to 5 anti-MM prior therapies and have MM that is refractory to previous treatment with at least 1 PI, at least 1 IMiD, and 1 anti- CD38 monoclonal antibody.

* Eastern Cooperative Oncology Group (ECOG) performance status of <= 2.

* Female participants of childbearing potential must agree to use dual methods of contraception and have a negative serum pregnancy test at screening, and male participants must use an effective barrier method of contraception if sexually active with a female of childbearing potential. For both male and female participants, effective methods of contraception must be used throughout the study and for 7 months for female and 4 months for male following the discontinuation of study treatment.

* Active plasma cell leukemia.

* Documented systemic amyloid light chain amyloidosis.

* Active central nervous system MM.

* Only for SVd arm: Greater than Grade 2 peripheral neuropathy or Grade >= 2 peripheral neuropathy with pain at baseline, regardless of whether or not the participant is currently receiving medication.

* Radiation, chemotherapy, immunotherapy, or any other anticancer therapy (including investigational therapies) <= 2 weeks prior to Cycle 1 Day 1 (C1D1). (Steroids are permitted up to 1 pulse of 40 mg per day for 4 days in the 2 weeks prior to C1D1).

* Active graft vs. host disease (after allogeneic stem cell transplantation) at C1D1.

* Ongoing clinically significant non-hematological toxicities from prior treatments that are Grade greater than (>) 2 at C1D1.

* Inadequate hepatic function defined as total bilirubin >= 2x upper limit of normal (ULN) (>= 3x ULN for participants with Gilbert's syndrome), aspartate transaminase (AST) >= 2.5x ULN, and alanine transaminase (ALT) >= 2.5x ULN.

* Inadequate renal function defined as estimated creatinine clearance of lesser than (<) 20 milliliter per minute (mL/min), calculated using the formula of Cockroft and Gault.

* Inadequate hematopoietic function defined as the following:

1. Absolute neutrophil count (ANC) < 1000/cubic millimeter (mm^3)

2. Platelet count < 75,000/mm^3

3. Hemoglobin (Hb) level < 8.5 g/dL

* Life expectancy of < 4 months, based on the opinion of the Investigator.

* Major surgery within 4 weeks prior to C1D1.

* Uncontrolled active infection requiring parenteral antibiotics, antivirals, or antifungals within 1 week prior to first dose.

* Active gastrointestinal dysfunction interfering with the ability to swallow tablets, or any gastrointestinal dysfunction that could interfere with absorption of the study treatment.

* Known active hepatitis A, B, or C infection; or known to be positive for hepatitis C virus RNA or hepatitis B virus surface antigen.

* Female participants who are pregnant or lactating.

* Known intolerance, hypersensitivity, or contraindication to glucocorticoid therapy at C1D1.

* Concurrent therapy with approved or investigational anticancer therapeutic including topical therapies.

* Prior exposure to a SINE compound, including selinexor.

* Serious, active psychiatric or active medical conditions which, in the opinion of the Investigator or the Sponsor, could interfere with the participation in the study.

* Contraindication to any of the required concomitant drugs or supportive treatments.