Does Oracle write to data files in begin/hot backup mode for DBA ?

Oracle will stop updating file headers, but will continue to write data to the database files even if a tablespace is in backup mode. In backup mode, Oracle will write out complete changed blocks to the redo log files. Normally only deltas (changes) are logged to the redo logs. This is done to enable reconstruction of a block if only half of it was backed up (split blocks). Because of this, one should notice increased log activity and archiving during on-line backups.

How does one backup archived log files for DBA ?

One can backup archived log files using RMAN or any operating system backup utility. Remember to delete files after backing them up to prevent the archive log directory from filling up. If the archive log directory becomes full, your database will hang! Look at this simple RMAN backup script:
RMAN> run {
2> allocate channel dev1 type disk;
3> backup
4> format '/app/oracle/arch_backup/log_t%t_s%s_p%p'
5> (archivelog all delete input);
6> release channel dev1;
7> }

What is strip sources generate options?

Removes the source code from the library file and generates a library files that contains only pcode. The resulting file can be used for final deployment, but cannot be subsequently edited in the designer.ex. f45gen module=old_lib.pll userid=scott/tiger strip_source YES output_file.

What are the different file extensions that are created by oracle reports?

Rep file and Rdf file.

Explain about stacked canvas views?

Stacked canvas view is displayed in a window on top of, or "stacked" on the content canvas view assigned to that same window. Stacked canvas views obscure some part of the underlying content canvas view, and or often shown and hidden programmatically.