Concept:In the correct electrochemical series, magnesium is more reactive than aluminium, so Mg comes before Al.
Explanation:The given order is Na, Ca, Al, Mg, Zn, Fe, Pb, H, Cu, Hg, Ag, Au.
The correct order should place Mg before Al: Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au.
Therefore, Al and Mg must be interchanged.
Answer:A. Al and Mg