Parallel construction means that you

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

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. give both sides of the argument in your essay.

A parallel contruction means generally that you use the same constructions, for example the same grammatical constructions or the same sentence structure repeatedly,

This can be in the headings, but it can also be in other parts of the text.

The correct answer here is B - but another answer, " use the same grammatical construction throughout the text" would also be correct.