Which step is necessary to create a 1-to-1 relationship between Entity A and Entity B?

Creating a 1-to-1 relationship between Entity A and Entity B requires that each instance of Entity A can be linked to only one instance of Entity B and vice versa. To achieve this, one effective method is to set up a reference directly, meaning that the identifier attribute of one entity needs to reference the identifier of the other entity.

Setting the data type of the identifier attribute of Entity B to Entity A Identifier establishes that each record in Entity B is specifically associated with one unique record in Entity A. This ensures that the linkage is enforced at the database level, allowing the application to maintain a strict 1-to-1 relationship as required.

In contrast, adding a new Entity C (as proposed in other choices) introduces complexity and additional tables, which is unnecessary for forming a direct 1-to-1 relationship. Moreover, adding a reference attribute to Entity A or creating a composite identifier would also complicate the relationship instead of simplifying it.

Thus, the approach of setting the identifier attribute in one entity to reference the other entity's identifier is a straightforward and effective way to establish a direct 1-to-1 relationship.