Choice A is the best answer because it uses a comma to effectively subordinate the list of varied fields in which transportation planners major.
Choice B is not the best answer because the comma and coordinating conjunction “and” result in an ungrammatical sentence.
Choice C is not the best answer because the coordinating conjunction “and” along with the subordinating conjunction “which” result in an ungrammatical sentence.
Choice D is not the best answer because is it unclear from this construction to what exactly the subordinating conjunction “which” refers.
