Which halogen does not show variable oxidation state?

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

Chlorine was discovered by

Which of the following is prepared by the electrolytic method?

Combination of which parameters explains the higher oxidizing strength of $F_2$ as compared to $Cl_2$ in water?

$X_2$ reacts with water to form $HX_{(aq)}$ and $HOX_{(aq)}$. $Y_2$ reacts with water to form $O_{2(g)}$,$Y^{-}_{(aq)}$ and $H^{+}_{(aq)}$. $Z_2$ does not react with water. $X_2, Y_2, Z_2$ are respectively