Which one is a redox reaction?

Correct answer: d Combustion

In combustion oxygen takes electrons and the other element or compound gives them up; so combustion is redox.

Why the other options are wrong

• a Neutralization: In neutralization an acid and a base give a salt and water; no electrons are exchanged, so it is non-redox.
• b Isomerization: In isomerization the atoms only rearrange inside the molecule; the book puts it in neither the redox nor the non-redox class.
• c Polymerization: In polymerization countless small molecules join into a big one; the book puts it in neither the redox nor the non-redox class either.