Step 1: Understanding oxidation tendencies.
Oxidation is the process of losing electrons, and a metal's tendency to undergo oxidation is often measured by its standard electrode potential. The more negative the electrode potential, the higher the tendency of the metal to lose electrons and undergo oxidation.
Step 2: Analyzing the options.
(A) Mg: Magnesium has a highly negative electrode potential, indicating that it has a high tendency to undergo oxidation.
(B) Ag: Silver has a positive electrode potential and is less likely to undergo oxidation.
(C) Al: Aluminum also has a relatively negative electrode potential, but not as negative as magnesium.
(D) Fe: Iron has a negative electrode potential, but its tendency to oxidize is less than magnesium's.
Step 3: Conclusion.
Magnesium (\( \text{Mg} \)) has the highest tendency to undergo oxidation, so the correct answer is (A).