Which of the following chemical reactions is an oxidation-reduction reaction?
a. cuso4 + bacl2 yields baso4 + cucl2
b. pb(no3)2 + 2nacl yields pbcl2 + 2nano3
c. n2 + 3h2 yields 2nh3
d. k2so4 + bacl2 baso4 + 2kcl

C. N₂ + 3H₂ → 2NH₃
In a redox, or oxidation-reduction, reaction, one of the reactants must be reduced, which means its oxidation number decreases, while the other reactant must be oxidized, which means its oxidation number increases. The oxidation number of nitrogen in the reactants is 0 and is -3 in the products, so it is reduced. Similarly, the oxidation number of hydrogen is 0 in the reactants but it is +1 in the products.

Pretty sure it's d. heterogeneous is separated by physical properties.