Final answer:
The serous membrane that lines the outer surface of a lung is the visceral pleura, which is part of the two-layered pleura that includes both the visceral and parietal pleura.
Step-by-step explanation:
The serous membrane that lines the outer surface of a lung is called the visceral pleura. The pleura consists of two layers: the visceral pleura that is superficial to the lungs and the parietal pleura which is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm.
The space between these two layers is known as the pleural cavity, which contains serous fluid that reduces friction during respiration.