A self join is a regular join, but the table is joined with itself. This kind of join can take the form of an inner join, a left outer join, or even a full outer join depending on the requirement.
* A. The join key column must have an index. (Incorrect)
While indexes can improve the performance of joins by reducing the cost of the lookup operations, they are not a requirement for a self join. A self join can be performed with or without an index on the join key columns.
* B. It can be a left outer join. (Correct)
A self join can indeed be a left outer join. This is useful when you want to include all records from the 'left' side of the join (the table itself), even if the join condition does not find any matching record on the 'right' side (the table itself again).