Which term would best fit the description: 'to make worse'?

The term that best fits the description 'to make worse' is indeed 'exacerbate.' This word specifically refers to the act of intensifying or worsening a situation, condition, or problem. For instance, one might say that certain environmental policies exacerbate issues of pollution and climate change, meaning they contribute to worsening these problems.

In contrast, the other choices do not convey the same meaning. 'Mitigate' refers to the act of making something less severe or serious, which is the opposite of making it worse. 'Rebuff' means to reject or snub someone or something, and it does not relate to the concept of worsening a situation. 'Subsides' refers to a decrease or diminishing in strength or intensity, which likewise does not align with the idea of making something worse. Thus, 'exacerbate' is the word that accurately captures the intended meaning.