Which halogen does not show variable oxidation state?

Arrange the following in the order of property indicated for each set:
$(i)$ $F_{2}, Cl_{2}, Br_{2}, I_{2}$ - increasing bond dissociation enthalpy.
$(ii)$ $HF, HCl, HBr, HI$ - increasing acid strength.
$(iii)$ $NH_{3}, PH_{3}, AsH_{3}, SbH_{3}, BiH_{3}$ - increasing base strength.

$X_2$ is used in the refining of $Ti$ metal by the van Arkel method. $Y_2$ does not liberate $O_2$ from water and does not form $HY$ and $HOY$ with water. $X_2$ and $Y_2$ are respectively:

The bond dissociation energy is highest for

Which of the following pairs is not correctly matched?

Which of the following statements is not true for halogens?