Q.Choose the most correct option.
Which of the following is not an example of redox reaction?
a. CuO + H2 → Cu + H2O
b. Fe2O3 + 3CO → 2Fe + 3CO2
c. 2K + F2 → 2KF
d. BaCl2 + H2SO4 → BaSO4 + 2HCl

Before oxidation numbers or electrons were used to define redox reactions, chemists used substance-based definitions. Oxidation, classically, is combination with oxygen (carbon burning to CO2) or, more broadly, combination with any electronegative element (magnesium reacting with fluorine or sulfur), and it is equally seen as removal of hydrogen (H2S losing hydrogen to become sulfur) or removal of an electropositive element (Hg2Cl2 losing mercury to become HgCl2). Reduction is the exact mirror image: removal of oxygen (Fe2O3 losing its oxygen to give metallic iron), removal of an electronegative element (FeCl3 losing chlorine to give FeCl2), addition of hydrogen (ethene being hydrogenated to ethane), or addition of an electropositive element (HgCl2 gaining mercury from SnCl2 to become Hg2Cl2). A reaction is redox precisely when both an oxidation and a reduction happen together, as in HgCl2 being reduced while SnCl2 is simultaneously oxidised. From these substance-based patterns come the definitions of oxidant/oxidising agent (a reagent that is itself reduced while oxidising something else) and reductant/reducing agent (a reagent that is itself oxidised while reducing something else) -- roles that later sections redefine, more generally, in terms of electrons and then in terms of oxidation number.