In the voltaic cell :
at the cathode ⇒reduction reaction
at the anode ⇒ oxidation reaction
We can see which metal is more reactive between Ag and Mg with voltaic series
Li-K-Ba-Ca-Na-Mg-Al-Mn- (H2O) -Zn-Cr-Fe-Cd-Co-Ni-Sn-Pb- (H) -Cu-Hg-Ag-Pt-Au
From the series , Mg more reactive than Ag, so that Mg undergoes an oxidation reaction and Ag reduction reaction
Half-reactions
cathode : Ag⁺ (aq) + e⁻ ⇒ Ag (s)
anode : Mg (s) → Mg²⁺ (aq) + 2e−
So the answer for :
15. A. Ag⁺ (aq) + e⁻ ⇒ Ag (s) (Reduction reaction)
16. C. Mg (s) → Mg²⁺ (aq) + 2e− (Oxidation reaction)
17. A. the anode (oxidation reaction)