That's a wonderful question : GMAT usually tests splits among because, in that, and -ing modifer.
Because - answers why?
in that and -ing modifier - answers how ?
As we have split between because and in that we have to choose between how or why.
1. Why B is an unusual art form ?
Just try to relate nature of above question with question : why are you fat/blind/deaf.. ?, does it makes sense : NO. expected sensible question would be how you got fat/blind/deaf... Similarly asking Why B is unusual is not a sensible question in itself.
2. How B is an unusual art form ?
As explained above answer of how is required in above context.
In that limit the context of answer to how B is wonderful. in what ways B is wonderful.
Understanding above explanation, we can eliminate A,B, and C.
Between D and E, E is correct.
Bricolage is an unusual art form
because it is formed of materials such as knickknacks and cast-offs not usually employed by fine artists.
A because it is formed of materials such as knickknacks and cast-offs
B because it is formed of materials like knickknacks and cast-offs that are
C because it is formed of materials, like knickknacks and cast-offs, materials
D in that its formation, materials such as knickknacks and cast-offs, is
E in that it is formed of materials such as knickknacks and cast-offs, materials