A coelom is a fluid-filled body cavity surrounded by the mesoderm. This cavity is present in many animals, including humans, and serves various functions such as providing a space for organs to develop and move independently, acting as a hydrostatic skeleton, and enabling efficient circulation of nutrients and waste products. The mesoderm, one of the germ layers in embryonic development, gives rise to various structures including muscles, bones, and connective tissues. Therefore, the correct answer is "A fluid-filled body cavity surrounded by the mesoderm."