Which one of the following is a redox reaction?

Al3 + (aq) + PO43 - (aq) → AlPO4 (s)

NaCl (aq) + AgNO3 (aq) NaNO3 (aq) + AgCl (s)

3Mg (s) + 2FeCl3 (aq) → 3 MgCl2 (aq) + 2Fe (s)

LiCl (aq) + AgCH3CO2 (aq) → AgCl (aq) + LiCH3CO2 (aq)

HF (aq) + KOH (aq) → H2O (ℓ) + KF (aq)

3Mg (s) + 2FeCl3 (aq) → 3 MgCl2 (aq) + 2Fe (s)

Explanation:

Redox reaction is a reduction-oxidation reaction in which oxidation states of atoms involved in the reaction are changed. The elements that receive electrons are considered to be reduced while the element that donates the electrons is considered to be oxidized. In the case of this reaction, Fe in the FeCl₃ is reduced into Fe by gaining 3 electrons - while Mg is oxidized to MgCl₂ because it donates 2 electrons.

If you check the moles of Mg, FeCl₃, Fe, and MgCl₂; you'll get to notice that the exchange of electron is balanced at 6.