What is the strongest symbol in “Once Upon a Time..” by Nadine Gordimer?

The Wall

Explanation:

In the frame story, the author Nadine Gordimer is asked to tell a childerns' story but she refuses to do that because of her artistic freedom so she decides to tell the story about one married couple.

That married couple was a wealthy one and they lived in a walled-off suburb. They had a son and they were afraid of ''people of another color''. They were afraid that they will come into their home. Because of that, they had installed the thing that later on killed their son and that was barbed wire.