Parallel construction refers to a repeated usage of the same grammatical structure. This can be in the heading or in the text. The correct answer is c. use the same grammatical construction in all similar headings throughout an outline, although option D comes close to it- if by "same way" you mean the same grammatical construction.
Parallel construction means that you a. give both sides of the argument in your essay. 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. begin each paragraph of your essay in the same way.