You have to look at the oxidation states of each atom in the reaction. Since redox reactions involve the transfer of electrons from one element to another, 2 elements need to have oxidation state changes.
option a is the only option that has a change in oxidation sates. Fluorine goes from 0 to -1 indicating that it gained electrons (got reduced) while bromine goes from -1 to 0 indicating it loses electrons (got oxidized).
(2Br⁻→Br₂+2e⁻ and F₂+2e⁻→2F⁻).
I hope this helps. Let me know if anything is unclear.
