The reaction occurs in which of the given galvanic cell?
$$\dfrac{1}{2}H_2(g) + AgCl(s) \rightleftharpoons H^+(aq) + Cl^-(aq) + Ag(s)$$

The reaction under consideration is $$\frac{1}{2}H_2(g) + AgCl(s) \rightleftharpoons H^+(aq) + Cl^-(aq) + Ag(s)$$.

The oxidation half‐reaction at the anode is $$\frac{1}{2}H_2(g) \rightarrow H^+(aq) + e^-$$ and the reduction half‐reaction at the cathode is $$AgCl(s) + e^- \rightarrow Ag(s) + Cl^-(aq)$$.

To assemble the galvanic cell, a platinum electrode is placed in contact with H₂ gas and H⁺ ions at the anode, while AgCl is in contact with Ag metal and Cl⁻ ions at the cathode. An HCl solution serves as the electrolyte, providing both H⁺ and Cl⁻ ions needed for the reactions.

The proper cell notation is $$\text{Pt} \,|\, H_2(g) \,|\, HCl(\text{sol}^n) \,|\, AgCl(s) \,|\, Ag$$ which corresponds to Option A.

Option C is not suitable because AgNO₃ would supply NO₃⁻ ions instead of the required Cl⁻, and Option D fails to provide H⁺ at the anode when using KCl. The correct answer is Option A.