Teratomas are unusual forms of cancer because they are composed of tissues such as tooth and bone not normally found in the organ in which the tumor appears.
(A) because they are composed of tissues such as tooth and bone
(B) because they are composed of tissues like tooth and bone that are
(C) because they are composed of tissues, like tooth and bone, tissues
(D) in that their composition, tissues such as tooth and bone, is
(E) in that they are composed of tissues such as tooth and bone, tissues

Teratomas are unusual forms of cancer because they

What is "they " talking about??? Teratomas or forms...hmmmmmm!!!

A and B definitely don't do that ...
Moreover we need "such as" as we are introducing examples...

Thus C is out.

Between D and E

D seems as if tooth and bones are the compositions..

Thus E

E) in that they are composed of tissues such as tooth and bone, tissues