The greenhouse gas that is most important when it comes to retaining a variety of longwave radiation is Water Vapor or H₂o.
There are two greenhouse gases that are known to absorb longwave radiation are Carbon Dioxide and Water Vapor.
Water Vapor is however considered to be more important than Carbon Dioxide because it absorbs more of the longwave radiation.
Find out more on Water vapor as a greenhouse gas at https://brainly.com/question/14205403.
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