To solve this problem, we need to analyze each statement to identify the incorrect one.$\newline$Let's evaluate each option:$\newline$• Option 1: The transition metals and their compounds are known for their catalytic activity due to their ability to adopt multiple oxidation states and to form complexes.$\newline$This statement is correct. Transition metals can change oxidation states and form complexes, which are key factors in their catalytic activity.$\newline$• Option 2: Interstitial compounds are those that are formed when small atoms like H, C, or N are trapped inside the crystal lattices of metals.$\newline$This statement is correct. Interstitial compounds involve small atoms fitting into the interstices of metal lattices.$\newline$• Option 3: The oxidation states of chromium in $CrO_4^{2-}$ and $Cr_2O_7^{2-}$ are not the same.$\newline$This statement is incorrect. In both $CrO_4^{2-}$ (chromate) and $Cr_2O_7^{2-}$ (dichromate), the oxidation state of chromium is +6.$\newline$• Option 4: $Cr^{2+} \, (d^4)$ is a stronger reducing agent than $Fe^{2+} \, (d^6)$ in water.$\newline$This statement is correct. $Cr^{2+}$ is a stronger reducing agent compared to $Fe^{2+}$ due to its higher tendency to lose electrons and form $Cr^{3+}. \newline$Therefore, the incorrect statement is Option 3, as the oxidation states of chromium in $CrO_4^{2-}$ and $Cr_2O_7^{2-}$ are indeed the same.