Answer : 3. "Data dictionary and transaction log "
Explanation :

During DBMS operation, essential files include the data dictionary, containing metadata like table structures, and the transaction log, recording all database transactions. The data dictionary guides the DBMS in understanding database organization, while the transaction log ensures data integrity by recording modifications. Both files are crucial for effective database management, enabling accurate querying, data manipulation, and system recovery in case of failures, ensuring the system's reliability and consistency.