To determine which statement is not true for halogens, let's analyze each option:$\\$Option 1: $$All but fluorine show positive oxidation states.$\\$• Fluorine is the most electronegative element and always exhibits an oxidation state of $-1. \\$• Other halogens (Cl, Br, I) can exhibit positive oxidation states, such as $+1, +3, +5,$ and $+7. \\$• Therefore, this statement is true.$\\$Option 2: $$All are oxidizing agents.$\\$• Halogens have high electronegativity and a strong tendency to gain electrons, making them good oxidizing agents.$\\$• Therefore, this statement is true.$\\$Option 3: $$All form monobasic oxyacids.$\\$• Halogens form oxyacids like hypochlorous acid (HOCl), hypobromous acid (HOBr), and hypoiodous acid (HOI), which are monobasic.$\\$• Therefore, this statement is true.$\\$Option 4: $$Chlorine has the highest electron-gain enthalpy.$\\$• Fluorine has a higher electron-gain enthalpy than chlorine, but due to its small size, the electron-electron repulsion is higher, making chlorine's electron-gain enthalpy slightly more negative.$\\$• Therefore, this statement is true.$\\$Conclusion:$\\$The statement that is not true is Option 1, as it is actually a true statement.$\\$The correct answer is Option 1.