Oxidation-reduction reactions require a change in the oxidation states of at least two elements. The equation that is NOT REDOX will not have any changes in any of the oxidation states.
Going from elements to compounds (b) always involved redox. In exchanges (a) and (c) a subscript change on a component will usually indicate a REDOX, but it still needs to be checked.
“Swapping” partners (a) often are not REDOX, but again, they need to be checked. So, to justify your selection (correct) identify which two elements are changing oxidation states. One will always go up (oxidize) equally to the reduction of another.
In (b) the element Ba is oxidized to