Step 1: Understanding the word "alleviate."
"Alleviate" means to make something less severe or more bearable. It is often used in the context of easing pain, hardship, or difficulty.
Step 2: Analyzing the options.
- Swell refers to something increasing in size or volume, which does not fit the meaning of alleviating.
- Aggravate means to make something worse, which is the opposite of alleviating.
- Repel means to drive away or resist, unrelated to easing or reducing difficulty.
- Assuage means to make something less intense or severe, which is a synonym of alleviate.
Step 3: Conclusion.
"Assuage" is the most accurate synonym for "alleviate," as both refer to easing or reducing the severity of something.
Thus, the correct answer is 4. Assuage.
Final Answer:
\[
\boxed{\text{The correct answer is 4. Assuage.}}
\]