In a multi-user environment, a "Race Condition" occurs when two users query the same record at the same time, see that Is_Registered__c is false, and both proceed to update it. To prevent this, developers must implement Row-Level Locking.

In Salesforce SOQL, the FOR UPDATE (Option C) keyword is used for this exact purpose. When a query includes FOR UPDATE, the platform places an exclusive lock on the records returned. If a second transaction attempts to query the same records using FOR UPDATE, it will be forced to wait until the first transaction is completed (either committed or rolled back). If the lock isn't released within 10 seconds, the second transaction throws a QueryException.

By using FOR UPDATE, the first user who executes the query "reserves" the Account. The second user's request will wait, and by the time their query actually executes and retrieves the data, Is_Registered__c will have been updated to true by the first user, thus failing the if (!account.Is_Registered__c) check and preventing a duplicate registration. Option D is a manual "optimistic locking" approach that is less reliable than the platform's native locking mechanism. Options A and B do not address the timing issue of the initial check.