Option B, a cavity filled with fluid, is the right answer.
A coelom is a fluid-filled mass decay in bilaterally symmetrical mammals that is absolutely filled with derivatives of mesoderm. In simple terms, it is the main body cavity in many animals and is found inside the body to encompass and hold the digestive expanse and different organs. In matured creatures, it is marked with mesothelium and in other mammals like molluscs endures undifferentiated.