Which word is defined as the act of making a situation less severe?

The word "alleviate" refers to the act of making a situation less severe or more bearable. When used in context, it often relates to reducing pain, distress, or difficulty. For instance, one might alleviate suffering through various means, such as providing comfort or solutions to problems. This aligns perfectly with the definition provided in the question, emphasizing its suitability as the correct choice.

The other terms do not pertain to this definition. "Disparage" means to belittle or speak of someone or something in a disrespectful way, which does not connect to the idea of lessening severity. "Dissonant" describes a lack of harmony or agreement, often used in musical contexts but not in ways related to mitigation. "Appall" means to shock or horrify someone, which suggests an increase in negative feelings rather than a reduction. Therefore, "alleviate" stands out as the word that best fits the definition of making a situation less severe.