Option A is correct. Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
$2\text{AgCl} \xrightarrow{\text{sunlight}} 2\text{Ag} + \text{Cl}_2$
The silver formed makes AgCl appear grey.
This is a photodecomposition reaction — sunlight decomposes AgCl into silver (grey) and chlorine gas. The Reason directly explains why the grey colour appears (metallic silver deposits), so it is the correct explanation of the Assertion. Students often confuse this with option B; remember, whenever the R logically and directly causes A, choose option A.