Apex domains cannot have CNAME records.
Question and response
Question
dig nsg-static-122.87.75.182-airtel.com. NS
Answer
nsg-static-122.87.75.182-airtel.com. 5 NS ns15.abovedomains.com.
nsg-static-122.87.75.182-airtel.com. 5 NS ns16.abovedomains.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0