In the redox reaction AgNO3 + Na -> NaNO3 + Ag, which element has been oxidized

A. Silver (Ag)
B. Oxygen (O)
C. Sodium (Na)
D. Nitrogen (N)

Na

The sodium on the left is elemental sodium, the oxidation state is zero. The sodium on the right of the equation is the sodium ion, which is +1. The oxidation number has increased from zero to +1, which is oxidation. (Oxidation is the loss of electrons leading to a more positive oxidation state.)

Na has changed from a 0 charge to a +1 charged. It has lost an electron. Losing electrons is called oxidation.