To determine which statement about hydrogen is incorrect, let's analyze each option:$\\$Option 1: $$Hydrogen has three isotopes of which tritium is the most common.$\\$• Hydrogen indeed has three isotopes: protium $({}^1H),$ deuterium $({}^2H),$ and tritium $({}^3H). \\$• However, protium is the most common isotope, not tritium.$\\$• Therefore, this statement is incorrect.$\\$Option 2: $$Hydrogen never acts as cation in ionic salts.$\\$• In ionic compounds, hydrogen can form hydride ions $(H^-),$ but it can also form cations $(H^+)$ in some cases.$\\$• However, in most ionic salts, hydrogen is more commonly found as an anion.$\\$• This statement is generally considered correct in the context of typical ionic salts.$\\$Option 3: $$Hydronium ion, $H_3O^+,$ exists freely in solution.$\\$• In aqueous solutions, hydrogen ions $(H^+)$ associate with water molecules to form hydronium ions $(H_3O^+). \\$• These ions do exist freely in solution, making this statement correct.$\\$Option 4: $$Dihydrogen does not act as a reducing agent.$\\$• Dihydrogen $(H_2)$ is a well-known reducing agent. It can reduce metal oxides and other compounds.$\\$• Therefore, this statement is incorrect.$\\$Conclusion:$\\$The incorrect statement is Option 1, as it incorrectly identifies tritium as the most common isotope of hydrogen.$\\$The correct answer is Option 1.