In the reaction, k2so4(aq) + ba(no3)2(aq) → baso4(s) + 2 kno3(aq) , which ions are the spectator ions?

Answer is: spectator ions are NO₃⁻(aq) + 2K⁺.
Balanced reaction hemical reaction: 
Ba(NO₃)₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KNO₃(aq).
Ionic reaction: 
Ba²⁺(aq) + 2NO₃⁻(aq) + 2K⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2K⁺(aq) + 2NO₃⁻(aq).
Net ionic reaction: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
A spectator ion is an ion that exists as a reactant and a product in a chemical equation.

Ibelieve it is b. proton.hope my answer has you : )