Final answer:
Phosphorus (P) is the least malleable element in its solid state among the provided options, as it is a nonmetal and typically brittle.
Step-by-step explanation:
The least malleable element in its solid state among phosphorus (P), strontium (Sr), antimony (Sb), and copper (Cu) is phosphorus (P). Malleability is a property that generally increases as you move from the right to the left of the periodic table and from top to bottom within a group, as elements transition from nonmetals to metals. Malleable materials can be deformed under external pressure without cracking, a property that is typical of metals. Copper (Cu) is a metal known for its high malleability and ductility, strontium (Sr) is an alkaline earth metal with moderate malleability, and antimony (Sb) is a metalloid with properties between metals and nonmetals but still more malleable than nonmetals. Phosphorus (P), however, is a nonmetal and is generally brittle in its solid form, thus being the least malleable of the options provided.