I think D is the right answer.
They hide in places like driftwood and vegetation
The answer is; cell surface receptors that are recognized as antigens by the recipient's immune system.
Therefore the organ is attacked as a foreign substance . This is why organ transplants are given by the closest family member to reduce the antigen variability that causes transplant rejection. Tissue typing is done to ensure that the organ or tissue is as similar as possible to the tissues of the recipient.
They have bilateral symmetry and a rounded shape.
