Halogens exist in - 1 , + 1 , + 3 , + 5 and + 7 oxidation states. The halogen that exists only - 1 state is

Step-by-step solution

Here fluorine atom has no d-orbital in its valence shell, hence it cannot expand its octect and exhibit variable oxidation states, Being the most electronegative, it exhibits only – 1 oxidation state. Rest other here can do it as have d-orbital & can expand their octet states. Eg. B r F 3 + 3 , I F 7 + 7