Silver chloride (AgCl) is decomposed by sunlight into silver (grey) and chlorine gas. This is a photochemical decomposition reaction: $2AgCl \xrightarrow{sunlight} 2Ag + Cl_2\uparrow$
The examiner expects you to name the type of reaction (photochemical decomposition) and give the equation. The product silver (Ag) is grey, which explains the colour change. This is a standard example of a decomposition reaction triggered by light, not heat.