Which of the following statements is not true for halogens ?

Answer: 2. All but fluorine show positive oxidation states.

We need the statement that is NOT true for halogens.

• Option 1: Cl, Br, I and (as HOF) F each form a monobasic oxyacid, so this is accepted as true.
• Option 3: all halogens are oxidising agents, true.
• Option 4: chlorine has the most negative electron-gain enthalpy (more than fluorine, whose small size causes strong electron repulsion), true.
• Option 2: it says all halogens except fluorine show positive oxidation states. This is the statement the key treats as untrue, because fluorine does show +1 in HOF, so "all but fluorine" is not strictly correct.

The answer is option 2. Note: many textbooks say fluorine shows only \(-1\), so this question is open to debate; we follow the published key.