Final answer:
The true statement about outer joins is that the FULL, RIGHT, or LEFT keyword must be included to specify the type of outer join. These keywords determine which table's rows will be included even if there's no match in the other table.
Step-by-step explanation:
The statement about outer joins that is true is: d. The FULL, RIGHT, or LEFT keyword must be included.
Outer joins are used in SQL to return not only the rows that have matching values in both tables, but also those rows that do not have a match. The LEFT join includes all rows from the left table, and matched rows from the right table; if there are no matches, NULL values are used to fill in columns from the right table. Similarly, the RIGHT outer join includes all rows from the right table, and the matched rows from the left table. The FULL outer join returns all rows when there is a match in one of the tables, so it combines the effects of both left and right outer joins.
The other options are incorrect because the OR operator can indeed be used to link outer join conditions, the tables do not necessarily have to be aliased (it's encouraged for better readability), and outer joins are not always evaluated before other types of joins in the query.