Your snapshot questions have been answered many times in your other posts:
(There are more, but carefully reading through all of the above posts should help to refresh your memory.)
"I mean If I have 30 GB of D drive,Then Snapshot of 30 Gb (D') is also created during backup?"
It depends how much data changes.
Snapshots are not backups, if I have data on my disk and make a copy-on-write snapshot, it is 'instant, and no extra space is required, until I make a change to the data.
If you google for how snapshots work you will find some good explanations.