What r the process to apper for 2020 12th cbse bord exam if i passed class 11th in 2017.

Yes , as per CBSE guidelines it is possible but final decision lays in your college administrations hands. You have to explain a valid reason for this to your college if they are satisfied and are willing to take necessary steps then it is possible. However in one year gap you should stay in touch with your studies otherwise it may become a harmful decision in terms of results.