Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is the use of punctuation within a sentence. This choice correctly uses a comma to mark the boundary between the introductory subordinate clause ("With…windows") and the main clause ("transitional design splits the difference between old and new").
Why the other choices are wrong
Choice A
Choice A is incorrect because a semicolon can’t be used in this way to separate the subordinate clause ("With…windows") from the main clause ("transitional…new").
Choice B
Choice B is incorrect because a dash can’t be used in this way to separate the subordinate clause ("With…windows") from the main clause ("transitional…new").
Choice C
Choice C is incorrect because it results in a rhetorically unacceptable sentence fragment beginning with "with."