Teratomas are unusual forms of cancer because they are composed of tis

If you think of a simpler sentence, you might see the problem with answer A. Take this example:

The Eastville Zoo features unusual animals such as snakes not normally found in the local area.

You could interpret that sentence in two different ways. It could mean that snakes are found in the local area, but the snakes at the Zoo are unusual because they're types of snake that are not found in the area -- that is, it could mean "The Zoo features unusual animals such as {snakes not normally found in the local area}". Or it could mean "The Zoo features unusual animals, such as snakes, not normally found in the local area", which now means something completely different: snakes are unusual, and are not found in the local area at all.

Answer A is subject to the same two potential interpretations. Maybe tooth tissue is normally found in the organ where the teratoma is found, but the particular type of tooth tissue in the teratoma is not. Or maybe answer A has the same meaning as answer E, and the tissues themselves are not normally found in the organ. Since answer A is subject to two possible interpretations, both of which are at least vaguely plausible meanings, and answer E is unambiguous, we should prefer answer E here.