Parallel construction refers to a repeated usage of the same grammatical structure, so the best answer is this one: C. use the same grammatical construction in all similar headings throughout an outline. but it does not have to in the in the headings, in can also be in other parts!
Parallel construction means that you A. begin each paragraph of your essay in the same way. B. repeat the first paragraph of your essay as the last paragraph. C. use the same grammatical construction in all similar headings throughout an outline. D. give both sides of the argument in your essay