Parallel construction means that you

a. give both sides of the argument in your essay.

b. use the same grammatical construction in all similar headings throughout an outline.

c. begin each paragraph of your essay in the same way.

d. repeat the first paragraph of your essay as the last paragraph.

Parallel construction means the same grammatical construction used repeatedly many times in the same text. It could be in the headlines, but not neccessarily: it could also be in the text: the important part is that it's the same grammatical construction.

The correct answer is: b. use the same grammatical construction in all similar headings throughout an outline.