Among the elements listed, phosphorus (P) is generally the least malleable in its solid state. Phosphorus exists in several allotropes, with red phosphorus being more stable but generally non-metallic, leading to brittleness. In contrast, copper (Cu) and strontium (Sr) are known for their malleability, and antimony (Sb) displays some metallic characteristics but is also relatively brittle. Therefore, the answer is:
phosphorus (P).