Answer :
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!
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!