Which statement most accurately describes Surfactant?

A. A reflex that stabilises gaze during head movement by producing compensatory eye movement. B. A loop-of-Henle mechanism that establishes the medullary osmotic gradient for urine concentration. C. A renal hormone that stimulates red-cell production in bone marrow during hypoxia. D. A phospholipid-rich substance that lowers alveolar surface tension and helps prevent collapse.