This is the question when I go for an job interview. So I don't know the meaning of the two fields.
Since 1/a and 4/a have the same value "a", so these 2 rows are not unique; 2/b and 3/b are the same; 3/c and 3/b have the same value "3". So only 5/d is the unique row, because 5 and d are not duplicated.