To answer this question, we need to understand the SQL300 error code in SQL.
Option A) DDL not allowed on this table definition - This option is correct because the SQL300 error code typically indicates that a Data Definition Language (DDL) statement is not allowed on the specified table definition. This error can occur when trying to alter or modify the structure of a table in a way that is not permitted.
Option B) Column with this name already exists - This option is incorrect. The SQL300 error code is not related to a column already existing. Instead, it is specifically related to DDL statements on table definitions.
Option C) View's WITH CHECK OPTION validation failed in UPDATE - This option is incorrect. The SQL300 error code is not related to the validation failure of a view's WITH CHECK OPTION in an UPDATE statement. It is specific to DDL statements on table definitions.
Option D) One or more matching rows is locked by another user - This option is incorrect. The SQL300 error code is not related to locked rows by another user. It is specific to DDL statements on table definitions.
Therefore, the correct answer is A) DDL not allowed on this table definition.