Step 1: Definition of "alleviate."
"Alleviate" signifies reducing the intensity or making something more tolerable. It is commonly employed when referring to the easing of pain, suffering, or challenges.
Step 2: Examination of choices.
- "Swell" implies an increase in size or volume, which does not align with the concept of alleviating.
- "Aggravate" means to worsen a condition, directly contradicting the meaning of alleviating.
- "Repel" denotes pushing away or resisting, unrelated to the reduction of difficulty.
- "Assuage" signifies lessening the intensity or severity of something, making it a synonym for alleviate.
Step 3: Determination. "Assuage" is the most fitting synonym for "alleviate" due to their shared meaning of easing or diminishing the severity of a situation.Therefore, the correct selection is 4. Assuage.
Final Determination: [\boxed{\text{The correct answer is 4. Assuage.}}]