The sentence as written contains no grammatical errors. So substantial has been X that Y is an accepted construction. Other acceptable forms would be So substantial has X been that Y, or X has been so substantial that Y. There is no reason for the correct answer to switch to one of these other forms, but be aware that it could do so.
A quick scan of the choices doesn't reveal any useful grouping. Proceed to take a closer look at the choices.
(B) creates a redundancy by using "so" twice for the same purpose. (B) would have been fine had it said, The increase in demand for treehouses has been so substantial that.... As is, however, (B) must be eliminated.
(C) makes no sense when read into the sentence: "The increased demand ... has been such, the world's leading...." The sentence is now missing a connector between the first clause and the second. Eliminate (C).
(D) and (E) use "such ... has been" incorrectly. The phrase such has beenis used to mean this is how things are, not to convey that the quantity of something is great enough to lead to a certain result. For example, the phrase is used correctly in Such has been my life, or Such has been the history of this country. With this question, however, the sentence is trying to say that the increase in demand has been great enough to lead to much more work for the leading builder. "Such ... has been" is therefore inappropriate here. Eliminate (D) and (E).
Posted from my mobile device _________________
Everything will fall into place…
There is perfect timing for
everything and everyone.
Never doubt, But Work on
improving yourself,
Keep the faith and
Stay ready. When it’s
finally your turn,
It will all make sense.