I agree completely with Toreador on this - the query returns the two table names and the name of the foreign key constraint between them.
As the first two answers seem to mean the same thing it came down to the fine details:
"The foreign key name [that] exists between the source and destination tables."
The name of the foreign key constraint is what the query returns - so this was my answer.
"Shows the relationship that exists in each table and the foreign key name used in the relationship"
No the query doesn't return the relationship. it shows that they are connected, but not how which to me requires knowledge of which columns relate to each other and perhaps the on-delete and on-update actions. So to me this answer is wrong.
But that is not what the official answer is - am I misinterpreting the meaning of the words?