The solubility of AgCl is decreased by adding which of the following?

NaCl

NH₃

AgNO₃

None of these

Adding NaCl increases the concentration of Cl⁻ ions, which shifts the equilibrium of $AgCl leftrightarrow Ag^+ + Cl^-$ .

This is an example of the common ion effect, which decreases the solubility of a sparingly soluble salt.