Reasoning for redox reactions to be classified as such Could anyone help me out solving this question regarding redox reactions? I am fairly new to this topic.

Which of the following is a redox reaction?
(1) $\ce{NaCl + KNO3 -&gt; NaNO3 + KCl}$
(2) $\ce{CaCO3 -&gt; CaO + CO2}$
(3) $\ce{Mg(OH)2 + 2NH4Cl -&gt; MgCl2 + 2NH4OH}$
(4) $\ce{Zn + 2AgCN -&gt; 2Ag + Zn(CN)2}$

I know that the answer here is option (4) but could someone help me understand why this is so? I see a change in oxidation in the other three equations but it is not redox.
$\ce{Zn + 2AgCN -&gt; 2Ag + Zn(CN)2}$
Zinc changes its oxidation state from $0$ to $+2$, and silver changes its oxidation state from $+1$ to $0$. So, zinc is reducing agent and silver is oxidising agent.
$\ce{Zn^0 -&gt;[-2e^-] Zn^{+2}}$ (oxidation);
$\ce{2Ag^{+1} -&gt;[+2e^-] 2Ag^0}$ (reduction).
Other reactions are just equations of ion exchange without changing the oxidation states of chemical elements.