Option 1 -> Swell means to become larger or expand, which is unrelated to reducing severity.
Option 2 -> Aggravate means to make worse or more severe, which is the opposite of alleviate.
Option 3 -> Repel means to drive away or resist, which doesn't relate to lessening intensity.
Option 4 -> Assuage means to make less severe or to soothe, which directly matches the meaning of alleviate.
Hence, Option 4: Assuage -> Both "alleviate" and "assuage" mean to make something (such as pain, distress, or a problem) less intense or more bearable -> correct