Over a 100-year period, nitrous oxide has a much higher global warming potential per unit mass than carbon dioxide or methane, so each kilogram emitted traps far more heat.
Option A:
Incorrect; carbon dioxide is used as the reference gas with a GWP of 1 and has lower GWP per unit mass than nitrous oxide, even though its total impact is large due to volume emitted.
Option B:
Incorrect; methane has higher GWP than CO₂ but still lower than nitrous oxide on a 100-year horizon.
Option C:
Incorrect; although water vapour is an important greenhouse gas, it is not usually assigned a standard GWP in this way and is strongly controlled by temperature.
Option D:
Correct; nitrous oxide has the highest GWP among the listed gases over a 100-year time frame.
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