I would say (b) because this answer is the most focused. It's also a topic which would easily fit in a five paragraph essay, while the others would most likely require extensive research and would only fit in an at least ten page paper.
Briefly tie up what your introduction said.
DO NOT add any new information.
You can add what you have learned however that should be covered before the conclusion.
The writer can also propose a question to its readers.
A - accordingly
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