A teratoma is a cancerous tumor that contains tissue that is like:

A) an embryo.
B) brain tissue.
C) long bones.
D) thymus gland.

A teratoma is a tumor that contains tissues that are similar to the three germ layers of an embryo, which are the ectoderm, mesoderm, and endoderm. These layers give rise to different tissues and organs in an embryo, so a teratoma can potentially contain various tissues and structures such as skin, hair, teeth, bone, muscle, and even more complex organs like eyes or limbs. Therefore, A is the best choice as it describes the embryonic nature of a teratoma. B, C, and D are not accurate as they do not encompass the full range of possible tissues found in a teratoma.