Answer: Type 2 pneumocytes.
Step 1: Type I pneumocytes form the thin alveolar wall across which gas exchange happens. Type II pneumocytes are the cuboidal cells that synthesise and secrete pulmonary surfactant.
Step 2: The chief surfactant component is dipalmitoylphosphatidylcholine (DPPC, also called dipalmitoyl lecithin). It lowers alveolar surface tension and stops alveolar collapse at end-expiration.
Step 3: In RDS, immature or damaged Type II pneumocytes cannot make enough DPPC, so surfactant is deficient. Alveoli collapse, work of breathing rises, and the infant develops respiratory distress.
Step 4: Bronchial epithelium is not the source of surfactant, so it is not the answer. Ref: Robbins Basic Pathology, 10e.