What is a coelom?

A coelom is indeed defined as a space that forms within the trunk during embryonic development. This body cavity is significant because it allows for the separation of organs from the body wall, facilitating more complex organ development and movement. The coelom also provides a protective cushion for internal organs and allows for their independent growth and functioning.

In contrast, the other options describe different anatomical structures or features. A muscular sheet in the thoracic cavity refers to the diaphragm, which plays a critical role in respiration but does not define a coelom. Similarly, a cavity containing the lungs pertains to the pleural cavity, which is closely associated with respiratory function. Lastly, the lining for the abdominal and pelvic cavities pertains to the peritoneum, which helps protect and support internal organs but is not the same as the coelom itself. Understanding the coelom's role in development and anatomy helps clarify the organization and evolution of body plans in multicellular organisms.