Which compound dissociates to produce the ions so42–(aq) and nh4+(aq)?

The compound that dissociates to produce the ions SO4^-2 (aq) and NH4+ IS (NH4)2SO4 (s). 
(NH4)2SO4 is an ionic compound that dissociates itself upon its addition to the solvet to form a solution. When it is in the solution it forms ions.
The dissociation of ammonium sulfate forms 2 moles of ammonium ions  (NH4+) and 1 mole of sulfate ion (SO4^2-).
That is;  (NH4)2SO4 = 2 NH4+ + SO4^-2

The teacher is mostly talking about the distillation of a mixture

27. =3,500,000 x 4,000 = 14,000,000,000