Which equation represents an oxidation-reduction reaction?

a. hcl + koh? kcl + h2o

b. 4hcl + mno2? mncl2 + 2h2o + cl2

c. 2hcl + caco3? cacl2 + h2o + co2

d. 2hcl + fes? fecl2 + h2s?

In an oxidation-reduction reaction there is an exchange of electrons.

The exchange of electrons implies change in the oxidation states: at least one element increases its oxidation number while other reduces it.

By simple ispection you can predict that in the equation b. there is a change in oxidation states of Cl and Mn.

Now you can check it:

Equation 4H Cl + Mn O2 - > Mn Cl2 + 2H2 O + Cl2

oxidation sates 1 + 1 - 4 + 2 - 2 + 1 - 1 + 2 - 0

The oxidation state of Cl in HCl is 1 - and it changed to 0 in Cl2

The oxidation state of Mn in MnO2 is 4 + and it changed to 2 + in MnCl2

Answer b.