Which one of the following reactions is an example of photodecomposition?

The correct option is D

$2\mathrm{AgCl}\left(\mathrm{s}\right)\stackrel{light}{\to }2\mathrm{Ag}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)$

Photo decomposition: It refers to a reaction in which the reactant is broken down into its constituent elements in the presence of light. It is also referred to as photolysis.

From the given examples, the decomposition of Silver chloride (AgCl) is an example of photo decomposition. AgCl decomposes in the presence of light to form Silver (Ag) and Chlorine gas $\left({\mathrm{Cl}}_2\right)$.

Thus, option D is correct.