Which of the following halogens does always show oxidation state -1 ?

The oxidation state represents the number of electrons gained or lost when an atom forms chemical bonds. Halogens typically achieve a noble gas configuration by gaining one electron, resulting in a -1 oxidation state.

Fluorine consistently exhibits a -1 oxidation state due to its maximum electronegativity and absence of d-orbitals, preventing positive oxidation states.

Other halogens (chlorine, bromine, iodine) can display positive oxidation states (+1, +3, +5, +7) when bonded to more electronegative elements like oxygen, as their d-orbitals enable octet expansion in compounds such as $\mathrm{HClO4}$ or $\mathrm{ClF}_{3}$.

Thus, only fluorine always maintains an oxidation state of -1.