Which of the following compounds is likely to participate in a precipitation reaction?

A precipitation reaction occurs when two aqueous solutions react to form an insoluble solid, known as a precipitate. In this case, AgCl is the correct choice because it is known to be insoluble in water. When solutions containing silver ions (Ag+) and chloride ions (Cl-) are mixed, AgCl will form as a precipitate due to its low solubility in water.

Sodium chloride (NaCl), hydrochloric acid (HCl), and ammonium chloride (NH4Cl) all consist of ions that remain soluble in water. NaCl dissociates into sodium ions (Na+) and chloride ions (Cl-), neither of which will form a precipitate when mixed with other soluble compounds in typical aqueous conditions. HCl, being a strong acid, dissociates completely in water, maintaining its ionic form, so it doesn't lead to a precipitate. Similarly, NH4Cl dissociates into ammonium ions (NH4+) and chloride ions (Cl-), neither of which will form an insoluble compound under regular conditions. Thus, AgCl is the only compound among the options that is likely to participate in a precipitation reaction due to its insolubility.