Not able to find RSS Feed option in drop-down menu for private boards.

Hi Vishwajit Shinde,
RSS is an authentication-less pull "notification" feed. Therefor it's not available for private boards.
To keep following updates in a private board, members can still subscribe to that board and then be notified via the existing community notification emails: http://community.lithium.com/t5/Community-FAQs/How-do-I-enable-users-to-subscribe-on-a-board-level/ta-p/6615