Which one of the following compounds has a garlic-like smell?

Among phosphorus compounds, phosphorus trioxide (P₂O₃) is known for its distinct garlic-like smell. It is a white, waxy solid formed when phosphorus burns in a limited supply of air. Chemically, phosphorus trioxide is the anhydride of phosphorous acid (H₃PO₃). When exposed to moisture, it reacts to form phosphorous acid, which retains some similar chemical properties but lacks the characteristic odor.

The garlic-like smell of P₂O₃ comes from partially oxidized phosphorus species, which contain P–H bonds. These bonds are responsible for the formation of phosphine (PH₃) and related compounds that possess a strong, unpleasant, and garlic-like odor. Even a small amount of phosphine produced during the reaction or decomposition of phosphorus trioxide contributes to this smell.

In contrast, phosphorus pentoxide (P₂O₅) is odorless. It is a white, crystalline solid that acts as a strong dehydrating agent. It forms when phosphorus burns in excess oxygen. Because it lacks P–H bonds, it does not release phosphine or produce any noticeable odor.

Phosphorous acid (H₃PO₃), though related chemically, is also generally odorless. It is a stable, non-volatile compound used as a reducing agent in chemical synthesis and industrial processes.

Therefore, the presence of P–H bonds in phosphorus trioxide leads to the formation of volatile and odorous compounds, giving it the typical garlic-like smell. This property distinguishes it from other oxides and acids of phosphorus, making P₂O₃ the only one among the given options with such a distinctive odor.