An innovative tool that is rapidly gaining in popularity, 3-D printers allow engineers to create customized three-dimensional objects, including million year old fossils and prosthetic limbs, at a lower cost and of higher quality than in the past.
A) 3-D printers allow engineers to create customized three-dimensional objects, including million year old fossils and prosthetic limbs, at a lower cost
B) 3-D printers allow engineers to create customized three-dimensional objects, including million year old fossils and prosthetic limbs, and they can do it at a lower cost
*)
"An Innovative tool that is".... This opening modifier should modify a singular noun. By looking at the options, (A) and (B) can be eliminated because they both (A) and (B) have plural noun "3-D printers".
*) Both Such as and Including can be used to introduce examples. So, there should be other errors which will help us to eliminate 2 options.
C) a 3-D printer allows engineers to create customized three-dimensional objects, such as million year old fossils and prosthetic limbs, at a lower cost
This does not have any pronouns to create any errors. The opening modifier is correctly referring to the singular "A 3-D printer". If you cross off the list of items, the sentence says "a 3-D printer allows engineers to create customized three-dimensional objects at a lower cost". This sentence makes much sense. So, this is correct.
D) a 3-D printer allows engineers to create customized three-dimensional objects, including million year old fossils and prosthetic limbs, engineers can do this at a lower cost
"Engineers can do this at a lower cost". What does "this" refer to? Also, I am a bit confused about the logic.
E) a 3-D printer allows engineers to create customized three-dimensional objects, such as million year old fossils and prosthetic limbs, which they can do at a lower cost
I am not quiet sure what "They" is referring to. "They" can refer to bunch of items, such as engineers and three-dimensional objects. It's confusing and illogical. So, eliminate E.
Answer C