Final answer:
The 'authid current_user' clause must be included in an autonomous transaction subprogram when declaring definer's rights.
Step-by-step explanation:
The 'authid current_user' clause must be included in an autonomous transaction subprogram when declaring definer's rights. With definer's rights, the subprogram executes with the privileges of the subprogram owner or definer. When the 'authid current_user' clause is not specified, the subprogram executes with the privileges of the invoker, which may not have the necessary privileges to perform certain actions.