In the future, we're going to need the actual diagrams. There's no way that thing is official.
Anyway, judging from the question and the OA, the "Heavy Line Segments" consist of the overall perimeter of the entire figure, not counting those three line segments in the interior.
That makes this a simple problem: the total of all line segments in the figure is 24, and we have to subtract those three in the center. The question gives us that information being 3, and so 24 - 3 = 21. C.
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