Ans. is 'a' i.e., Carbon dioxide Important indoor air pollutantsPOLLUTANTSSOURCESRespirable particlesTobacco smoke, Stove, Aerosol sprays.Carbon monoxideCombustion equipment, Stove, Gas heaters.Nitrogen dioxideGas cookersCigarettesSulphur dioxideCoal combustionCarbon dioxideCombustion, respirationFormaldehydeParticle board, Carpet adhesives, Insulation.Other organic vapours(benzene, toluene, etc)Solvents, adhesives, resin products, aerosol sprays.OzoneElectric arcing, UV light sourceRadon and "daughters"Building materialAsbestosInsulation, Fire proofingMineral fibersAppliances