In the redox reaction AgNO_3 + Na -> NaNO_3 + Ag, which element has been reduced?

$A {g}^{+} \rightarrow A {g}^{0} \downarrow$
Silver ion undergoes a change in oxidation state from $+ I$ to $0$. Since silver has undergo a DOWNWARD change in oxidation state, it has been formally reduced.
On the other hand, elemental sodium has gone from $N {a}^{0}$ to $N {a}^{+}$; that is, it has been OXIDIZED.