Explanation:

- Relief generally means a feeling of reassurance and relaxation following release from anxiety or distress, or assistance offered to those in need.

- Option 1: Anger - This is an emotion characterized by antagonism toward someone or something. It is unrelated to relief.

- Option 2: Aid - This is often used as a synonym for assistance or help, aligning closely with one aspect of relief as help given to those in need.

- Option 3: Emotion - This is a broad term encompassing various feelings like joy, sadness, anger, etc. It's not closely synonymous with relief.

- Option 4: Stress - This is typically a feeling of emotional or physical tension, almost the opposite of relief.

Option 2: Aid is the most suitable synonym for relief when considering its aspect related to assistance.