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.