The word "must" expresses a strong obligation or necessity, which is appropriate here because the sentence emphasizes the urgency and importance of taking decisive action against climate change. The tone of the passage is serious and urgent, so "must" best reflects the mandatory nature of the actions needed.
Might: Indicates possibility, not obligation - too weak for the context.
Could: Suggests an option or ability, not necessity - again, too soft for the message.
Would: Often used for hypothetical situations - doesn't convey the required sense of urgency or duty.
Hence, option 1 is correct.
Study Notes
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