The word "mitigate" means to make something less severe or harmful. In this context, it refers to reducing the negative effects of climate change - such as extreme weather and rising sea levels. Therefore, "mitigate" fits both grammatically and meaningfully.
Widen: means to make something broader or larger - the opposite of what's needed in the context of reducing negative impacts.
Inflate: refers to expanding or increasing - again, the opposite of reducing or lessening the effects.
Maximise: means to increase to the greatest amount - not appropriate when discussing the harmful effects of climate change.
Hence, option 1 is correct.