The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed by a two-step mechanism:

The two-step mechanism is a slow mechanism and a fast mechanism. When we combine them, the result is

2H₂O₂ (aq) ⇒2H₂O (l) + O₂ (g)

Explanation:

We know that the decomposition of hydrogen peroxide is catalyzed by iodide ion, which means that the iodide ion will react with the hydrogen peroxide. There is a slow mechanism and a fast one:

H₂O₂(aq) + I₋(aq) ⇒ H₂O(l) + IO₋(aq) this is the slow reaction

IO₋(aq) + H₂O₂(aq)⇒ H₂O(l) + O₂(g) + I₋ (aq) this is the fast reaction

If we cancel the same type of molecules and ions, the final result is:

2H₂O₂ (aq) ⇒2H₂O (l) + O₂ (g)

i'm assuming that the correct answer is indeed the first one/ "a"..