To determine which statements are NOT correct, let's analyze each statement:$\\$A. Hydrogen is used to reduce heavy metal oxides to metals.$\\$• This statement is correct. Hydrogen can reduce metal oxides like copper(II) oxide to copper.$\\$B. Heavy water is used to study reaction mechanism.$\\$• This statement is correct. Heavy water (D$_2$O) is often used in kinetic isotope effect studies to understand reaction mechanisms.$\\$C. Hydrogen is used to make saturated fats from oils.$\\$• This statement is correct. Hydrogenation is the process of adding hydrogen to unsaturated fats to make them saturated.$\\$D. The H-H bond dissociation enthalpy is lowest as compared to a single bond between two atoms of any element.$\\$• This statement is incorrect. The H-H bond has a high bond dissociation enthalpy compared to many other single bonds.$\\$E. Hydrogen reduces oxides of metals that are more active than iron.$\\$• This statement is incorrect. Hydrogen cannot reduce oxides of metals that are more active than iron, as these metals have a higher affinity for oxygen.$\\$Based on the analysis, the incorrect statements are D and E.$\\$Therefore, the correct option is Option 3: D, E only.